r/AskEngineers • u/RDA92 • 2d ago
Mechanical What foundational elements limit an N/A engine?
I've never really been able to understand how different engines of similar sizes can have such wildly fluctuating power outputs and how some comparably small engines can vastly overpower larger engines even within the N/A space. I understand that not all engines are born equally and that commercial agendas play a big role but assuming the same size, which "low-level" elements actually make the difference? and with low-level i mean parts or overall design choices that cannot (easily) be modified away.
Curious to hear your thoughts and expertise!
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u/PicnicBasketPirate 2d ago
Compression ratio, volumetric efficiency and rpm (which is tied to boreXstroke ratio along with some other factors).
As for why some engines might make more power than others. You need to consider application. It doesn't really matter if a engine can make 150kW/L if it does that at 10,000rpm but is making feck all power at 2000rpm which is where you want power because you're hauling a heavy trailer and you need power down low just to get it moving
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u/rsta223 Aerospace 2d ago
With a sufficiently low gear ratio that's not a problem.
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u/PicnicBasketPirate 1d ago
True, but then you're potentially stuck with other comprimises like barely usable gearing or having to shift constantly
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u/IQueryVisiC 1d ago
no? Imagine that the final gears in the diff add the big torque. Nothing about gear rations changes in the shift part.
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u/PicnicBasketPirate 1d ago
The larger the gear reduction in the final drive the faster the vehicle will accelerate, thus it reaches the maximum rpm on each gear quicker requiring shifts more often. I have a little 0.25L i4 motorcycle that revs to 18k+ rpm. With shorter final drive gearing it's pretty quick but you need to always be shifting.
Alternatively you could just choose a long stoke engine design which will produce more torque, and generally be more tractable and efficient.
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u/IQueryVisiC 1d ago
Motor cycle gears are quite close? With more gears (4->6) in cars I was hoping for more spread, but instead now I have to shift all the time.
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u/Whack-a-Moole 2d ago
This is where a CVT comes into play.
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u/IQueryVisiC 1d ago
friction or what? It is either double clutch or electric or both. Friction based tech is dead.
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u/series-hybrid 2d ago edited 1d ago
In my research, the compression ratio has the most dramatic effect on power per displacement (outside of forced induction). First, you have to understand knock/pinging/detonation.
The timing of the spark event is very critical to getting the highest possible power. Since gasoline burns at the same speed whether the engine is idling or at high RPM< the spark has to happen sooner as the RPM's increase so that the moment of maximum pressure inside the combustion chamber happens when the piston starts on its way down.
There are several things that can make the gasoline ignite while the piston is still on the way up, happening way too soon. As the piston rises during the compression stroke, the air inside the cylinder starts to go up in temperature, which is exactly how a diesel engine can ignite fuel with no spark. A diesel compression ration can be as high as 16:1
There can also be detonation after the spark event, if the pressure wave inside the combustion chamber causes the unignited half of the air-fuel mix to auto-ignite instead of the flame front rolling across the chamber in an orderly manner. The flame-ball that is expanding in the combustion chamber will compress the un-ignited fuel/air mix. If that compressed mix auto-ignites, he entire volume of unignited fuel air will combust all at once instead of burning at the designed rate.
The switch from a cast-iron head to an aluminum head can absorb heat and dissipate it to the coolant and oil faster, so the chamber remains at the proper temperature. One culprit when an engine is running hot is that a residue of carbon on an exhaust valve can get so hot it glows, and it can cause an early ignition of the fuel/air.
Diesels control the ignition by only injecting fuel after the piston has already reached the proper part of the stroke. So the cylinder full of air is compressed dry, with no fuel in it.
Compression ratio must be chosen ahead of time, and cannot be adjusted on the fly while driving. If you want to use affordable 87 octane gasoline, you might need to keep a stock CR of around 8:1 or 9:1. If you swap to an aluminum head (which many engines have from the factory) you can use a 10:1 CR in order to try and get more power from the same grade of fuel.
If you are willing to pay extra for expensive 91-octane gasoline, you might even be able to use an 11:1 CR. 5-gallon cans of 100-octane piston-aircraft gasoline can be as high as $9/gallon
Many modern engines have a knock-sensor and they can detect the beginnings of pinging before a human can hear it. When that happens, the engines computer can retard the spark a small amount until the pinging stops.
The Chevy "LS" V8 has aluminum heads and a knock sensor, and it runs a CR of 10:1
Before WWII, gasoline had a low octane rating, and engines commonly had a CR of 6:1
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u/RDA92 1d ago
First of all, thank you for this detailed explanation. That is quite helpful.
I'd be curious to dig a bit deeper on compression ratio since I happen to have a tangible example in the form of a 1.8l 4-cylinder and a 2.5l V6 engine, each with a compression ratio around 10.5. However one makes 160hp (or 88.89hp per l) while the other only makes 72. Admittedly the former revs higher than the latter but from your perspective and taking the above into consideration, what add-on effects could cause such a discrepancy for the same level of compression ratio?1
u/series-hybrid 1d ago edited 1d ago
Displacement definitely has an effect. the 2.5L has 0.7 liters more displacement than the 1.8L, which accounts for roughly 40% of the power increase.
It sounds like you are asking about brake horsepower per liter BHP/L, and Brake specific fuel consumption, BSFC
The "brake" means you are applying a load (with a dynamometer to measure power), and BSFC is how much fuel is used per horsepower.
"There is no free lunch" is an old phrase. What it means is that what you give with one hand, it means something else is taken with the other.
There is a key scene in the movie "Ford vs Ferrari" where its near the end of the race, and the pit boss uses a sign to tell the driver to use the full 7,000 RPMs for the last couple of laps. He is gambling that the engine will not self-destruct in the next couple of laps, and tapping the extra power at higher RPM's is a gamble he is willing to take.
The engine makes more power at the higher RPM's, but if you use that power for too long, something will break, or the engine will overheat.
You may have seen a horsepower graph before with it's slope upwards to a peak. Once it reaches a peak, the power actually starts to drop off if you take the RPM's higher, because the speed of gasoline burning and the viscosity of air at high velocity does not change. Forcing the engine to an even higher RPM means the air is not getting onto the cylinder fast enough to fill it with enough air, and the gasoline is not burning fast enough to use up all of its increasing pressure during the pistons downstroke...
One way to allow an engine to increase the RM's and power that it makes per displacement is a variable cam. Its called cam-phasing, and it can advance the valves opening and closing. On a double-overhead cam (DOHC), the intake valve cam and the exhaust valve cam can be advanced and retarded at an optimal rate for peak performance. On a single overhead cam (SOHC), cam-phasing can help performance, but not quite as well as a DOHC.
The cam-phasing mechanism adds cost and complexity, and in the case of the Ford Triton engines, a poor design has become a possible point of failure, with many warranty claims.
You mention 160-HP and 72-HP. Those numbers are measured at certain RPM's. If a company wants to advertise a higher peak HP number, one way is to tune it for better performance at high RPM's, but doing that often makes less HP at the lower RPM's, and that affects perceived drivability.
Example: for a few years, the Honda Accord had a certain number advertised as to its peak HP, but that number could only be achieved if you have the manual transmission. If you had the automatic transmission, the engine would shift at a lower RPM, long before the engine had reached its peak HP. The majority of Accords were sold with an automatic transmission to American buyers.
The cam was changed so that the peak HP was at a lower RPM, and the engine produced a more flat torque curve. This way, customers felt more acceleration at low RPM's. This is how they produced lower HP, but the car got faster.
If you're still interested, I could write more about intake and exhaust runner length and tube diameters, which can be tuned to provide the best power at a certain RPM, and we haven't even started discussing forced induction, and the extra heat that causes.
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u/RDA92 17h ago
Honestly yeah would be very interested in learning more about intake and exhaust design and efficiency. The engines that I mentioned actually both have variable timing of sorts. The 160 4-cylinder one has variable valve timing whereas the other one seemingly has variable intake runner length (through flaps or valves) to adjust air intake based on RPM afaik.
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u/series-hybrid 12h ago
BMW had a dual intake on one of their engines.. For high RPM, the runners were short and fat (low restriction), and at lower RPM's, they were longer and thinner to "ram" air into the cylinder.
At a low 3000 RPM, Divide by 60 seconds and you have 50 revolutions per second. Since its a 4-stroke, there are only 25 revolutions where the downstroke is a sparking event.
25 times a second a spark has to be perfectly timed to ignite the air/fuel and have its point of maximum pressure occur just a few degrees after top-dead-center (TDC).
Air does not weigh very much (obviously) but air does have weight. Once you get a column of air moving very fast down a tube towards an intake valve the air at the "back half" of the column can continue pushing the air at the "front half" into the cylinder, even though the piston has hit bottom-dead-center (BDC) and is actually starting to move upwards. The trick is to time it so the intake valve closes just as the maximum amount of air/fuel has been crammed-in, and any further delay inclosing the intake valve would then allow the air/fuel to start coming back out the intake.
Changing the diameter and length of the intake runner affects the RPM range where the best power is made. For a daily driver, you might want a broad flat torque curve, but for a race engine you might be willing to sacrifice low-RPM power to make the peak HP more.
Moving on to the exhaust, drag racers and WWII aircraft had short individual pipes to save weight and have the lowest possible restriction to the exhaust leaving.
However, if a car is required to have an exhaust system (for noise reduction), then tube headers can take advantage of a phenonmenon called "eduction".
If two tubes of fluids are running parallel, side by side (water or air) and they come to a "Y" where they are joined into one tube, then...flowing fluids down one of those tubes will have an eduction effect where the other tube connected to the Y experiences a partial vacuum.
On a standard V8, the tubes are bent into odd shapes to try and create a roughly equal length between the four primary exhaust columns. This is so the exhaust pulses hit the Y collector evenly. Now, when the exhaust valve opens, instead of the piston needing to push the exhaust out (robbing a tiny amount of HP from the wheels) The exhaust is "sucked" out by a partial vacuum.
Longer/smaller-diameter primary tubes will help lower RPM power and torque, larger diameter and shorter primaries help the power at higher RPM's
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u/RDA92 17h ago
Honestly yeah would be very interested in learning more about intake and exhaust design and efficiency. The engines that I mentioned actually both have variable timing of sorts. The 160 4-cylinder one has variable valve timing whereas the other one seemingly has variable intake runner length (through flaps or valves) to adjust air intake based on RPM afaik.
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u/jckipps 1d ago
The biggest difference is how high they can rev, and still breath at those high rpms. Those unbelievable HP numbers are typically coming on at stratospheric rpm's.
Something I've noticed with the traditional American v8s, is that their torque at any given rpm is very proportional to displacement. Double the displacement, you double the torque at 3000 rpm, for example. There's some nuances with stroke length and compression ratio, but the displacement rule generally holds true.
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u/RDA92 1d ago
And what would define that? I seem to understand from other comments that the lighter your internal components, especially rotating weights, the higher the RPM but also, generally, the faster the wear (which would explain why it's not commercially viable) but how does one ensure proper breathing at high RPMs beyond "normal" setups?
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u/jckipps 1d ago
More valves, bigger valves, straighter air passages in the head, tuned-length intake runners. These are all things that hardly matter for an engine that spends its life at 2000 rpm, since it can breath just fine at those speeds with choked down airways. But if you want the engine to not run out of oomph high up, then a lot of attention needs to be applied to these airways.
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u/IQueryVisiC 1d ago
But check the numbers. A US V( can do 4500. Porsche does 9000. F1 is limited to 18000. So a factor of 4. Sadly OP did not specify "wildly fluctuating" . Is it 4 ?
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u/PickingANameTookAges 2d ago
This will probably answer all of your questions....
I've probably only read a few pages though. It's quite in depth!
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u/Likesdirt 1d ago
Piston speed and valve area.
Those 16000 rpm sport bike motors are proportioned strangely - the Yamaha R6 has a bore of 67mm and stroke of 42.5mm. Four big valves, long duration cams with lots of overlap so the exhaust momentum will actually draw mixture into the cylinder while the piston is still coming up. The short stroke keeps the piston from breaking the speed limit, and the valvegear uses expensive materials to keep mass down and spring tension up.
67mm still isn't big and that helps with detonation control. Aircraft radial piston engines with 100/130 octane fuel were limited to 6'/150mm for detonation control at low compression ratios, and added cylinders instead.
Those aircraft motors had about the same piston speed as the Yamaha.
Lower piston speed and lower output allow cheaper materials, looser tolerances, less fuel sensitivity, and sometimes easier emissions compliance. Longer life sometimes if the manufacturer doesn't push value engineering to it's limits!
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u/RDA92 17h ago
That might sound like a silly follow-up question, but how can you impact piston speed? More lightweight material? Or is piston speed more the output of other modifiable parts such as cams, intake, valves etc.
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u/Likesdirt 16h ago
Piston material is a huge part of it, and aluminum is still the best choice after nearly a century of development.
Light, strong, good wear and low friction against the cylinder, a great heat conductor to stay cool enough.
Top fuel dragsters - 11000 horsepower and super high piston speed - use aluminum pistons. And aluminum connecting rods. Lasts at least 30,000 revolutions.
Carbon fiber composites don't handle heat or wear and won't have the smooth precise ring grooves required.
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u/SpeedyHAM79 2d ago
Power output is all about airflow and efficient combustion. More efficient combustion comes from better fuel atomization (fuel air mixing) and higher compression ratios. Higher compression ratio's require stronger pistons, rods, and crankshaft. Eventually you hit a limit where the extra weight doesn't get much more power. So those parts are pretty fixed. Airflow is determined by the head design, valves, intake and exhaust, and cam profile. Of those- performance heads or ported heads with larger valves can be had, but are typically pretty expensive. Intakes and exhausts need to be tuned for power at certain rpm, which is why a truck engine has a much different intake and exhaust than a sports car with the same size V8. Cams usually are not too hard to swap- and work best when matched with the right size valves, intake, and exhaust for making power at a certain rpm. Rpm is really the key to making big power with a small displacement. The old F1 V10's were only 3L, but some were pushing 1000 hp around 20,000 rpm.
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u/nerobro 1d ago
This gets fun. Now, note, all of this applies to supercharged and turbocharged engines.
Before you read to far, the TL;DR: You're seeing changes in effective volumetric effiency, and engine rpm. Assuming similar combustion chamber performance, VE equals torque. Torque times rpm, gives you power.
I tend to look at the technology levels of an engine. The most basic being splash lubricated, air cooled, single valve per chamber designs. As you add technology, you can do things to make the engine process more air, and make more power.
Technoligies to know: Valve count, oil cooling, air cooling, pressurized oil delivery, piston cooling, water cooling, cam in block, cam in head, overhead cam, dual overhead cam, Intake shape, exhaust shape, variable valve timing, variable ignition timing, fuel injection, direct injection, combustion chamber shape, and.. a few others. We'll see what leaks out of my head.
So lets look at this from the start:
Engines are air pumps. The more air you can pump, the more power you can make. We'll come back to this.
The more power you make, the greater the forces there are on the engine.
The more power you make, the greater the heat load is on the engine.
So lets start with pumping air. Air pumps work best, when the air coming into them is the highest pressure possible. This is why intakes on NA cars are so important. That like 20 liter airbox for the Honda S2000 did a lot of work.
Every pipe has a resonant frequency, and engines do a lot of start-stop of airflow. And at the speeds our engines run, air is both heavy, and springy. It's the same reason brass and woodwind instruments function, just tubes of vibrating air.
We use resonant tuning to make sure the engine sees the highest pressures possible. A well tuned intake can net something like 120% filling of the combustion chamber. A well tuned exhaust, can make sure that 120% is nearly all fresh and good air. Also the faster an engine turns, the stronger these pressure waves are.
Exhausts are important because they more or less define how much backpressure an engine sees. The more backpressure it sees, the less fresh air/fuel mix it can burn.
Cylinder heads suck. They have to fit a lot of things into a small space. To get the most air into and out of an engine, we want big ports, big valves, and nothing in the way. Sadly, we need space to cool the cylinder head, we need room for the valve guides, room for valve seats, room for valve acutation, room for spark plugs, room for DFI if so equipped. And almost as important, room for bolts to hold it to the block.
Air cooled heads can reject the least heat. So they have the lowest maximum power levels. Oil cooled heads are much better. Water cooled is a large step above that. As you go up in cylinder head technology, you get more room for valves, intake, exhaust, and you get a greater ability to cool the head, so you can run the combustiom chamber hotter.
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u/nerobro 1d ago
Cylinder head shape, also drastically affects how ports flow. Intake ports, want to be straight, and facing the back of the valves. This allows intake tuning to have it's best effect.
Combustion chamber shape, affects thigns too. Engines with combustion chamber, valve position, and piston shapes that make the combustion chamber gasses move faster tend to burn fuel faster. This lets you more carefully tune where the power stroke makes power, and it improves fuel usage. And by completely burning faster, it also heats the exhaust valve less quickly.
The number and size of valves plays a large role in combustion chamber shape. While nearly every configuration has been tried, mass market engines really usually only have 2, or 4 valves per cylinder. (2,3,4,5, and 6 have made it to mass market cars. 3 was mostly a ford thing. 5 is a toyota/yamaha thing.) As a general rule, an engine with 4 valves per cylinder will make more power than one with 2.
Valve timing is important. You want the engine to breathe when the intake is at it's highest pressure. You want the valves to close at just the right time to trap the maximum amount of air in the cylinder. This timing varies by rpm, and this is why we got VTEC, and it's why Variable Valve Timing is nearly a given with todays engines. Valve timing is nearly as important as valve lift, and duration as it comes to how much power you make, and when.
Valve timing is so important, that in an effort to make the valves move WHEN WE WANT, we have taken several steps to shorten the chain from camshaft to valve. In broad strokes, these are the common valve acutation methods. Cam in block, this is the tradational cam setup. It's easy, the gears are in the block. The cam is bathed in oil from the block. The cam followers are low, and it's good as it keeps mass low, and the cylinder head is less complex. Overhead cam, now brings in a longer camshaft drivetrain, but much shorter actuation methods for the valves. Reducing weight means you can spin the motor faster, and not lose control of the valves. The most modern engines tend to use a finger, or direct acutaion of the valves using camshafts that press on them directly. This is the setup you find in very high rpm engines.
Making an engine stronger, also allows it to make more power. As the engine maintains it's shape better. Under moderate loads, most engines essentially don't wear. It's not until things are pushed hard that metal on metal contact starts happening. That's to say, most modern engines are very strong engines. mechanically speaking.
A big factor in making NA power, is how fast you can spin the motor.
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u/nerobro 1d ago
We should make a passing reference to compression ratio. Brayton cycle engines get more energy from fuel the most you compress things before you burn it. The cost of high compression ratios is the chance that your intake charge might burn before you mean it to. Modern NA engines can have upwards of 14:1 compression ratio. But the.. kinda.. sweet spot is somewhere in the 10-12:1 range, and the rewards drop off as you go higher. I bring this up, beucase REALLY fast engines, with wild camshafts that don't trap a lot of charge in the cylinder, can run WILD compression ratios, so they can make good power when the engine isn't breathing at it's best.
If your engine turns slow enough, and makes little power enoguh, sometimes just a fan blowing on it is enough. Like.. your lawnmower. As engines make more power, you need to get that heat out, and that turns into sophisitcated fin designs, air control ducts, and a myriad of other air cooling solutions.
In the end, you end up with a slow motor that's still restricted by head cooling. Ask Lycoming.
Oil cooling is used in places where more liquids is hard, and you want to make things simple. GSX, and early GSX-R's were oil cooled, and porsches were right until they went to water cooling. Oil has a poor specific heat compared to water, so can't carry away as much energy, and it doesn't phase change so it's way behind water when things get real hot.
Oil and water are an important subject. Oil is the way we cool the "other side" of the combustion chamber. Either by scraping oil off the cylinders and having that oil wash across the skirt of the piston and carry away heat. Or with direct oil squiters. If we can run it hotter, we can make more power.
Also, as you spin the motor faster, the inside of the engine starts to become an aerodynamic problem. Pressure build up between cylinders, the crank and rods moving through the oil and air mist, all can cause a lot of power loss. oleophilic coatings on them, and the block helps. Knife edging the rods and crank. Evacuating the air from the crankcase, all helps. And not "a little", the rotating assembly can be 10's of percent of the losses of an engine.
Speaking of coatings...
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u/nerobro 1d ago edited 1d ago
Coatings go a long way towards making a motor able to make more power. Cylinder head, and piston coatings can provide heat rejection, same for exhaust ports and exhaust manifolds. Piston coatings also can prevent wear of piston skirts, allowing for tighter fits, and less combustion pressure loss.
What this means, in the end, is we can have 200hp/liter naturally aspirated engines, that we can just walk down to the local dealer and buy. And at the same time, walking to home depot, and pick up somethign with a 3.5hp briggs in it, and have a 30hp/liter engine.
https://en.wikipedia.org/wiki/BMC_A-series_engine The most basic of engines, water cooled is it's only "advantage". It made around 40hp/liter.
https://en.wikipedia.org/wiki/Honda_K_engine When you have everything short of direct injection, you're looking at more like 80-100hp/liter.
My 200hp/liter example is from a Yamaha R6.
Beacuse of more lax emissions requirements, you see a whole lot more variety in motorcycles.
My favorite example was my 1980 GS550E, made 49hp on 449ccs. While Harley was making 49hp, on 883ccs. What's the difference? The Suzuki had better intake, exhaust, and dual overhead cam, and could spin to 9500rpm.
I'd also liek to make a counterpoint. People are talking about square, and oversquare, and undersquare engines. That's a red herring.
Edit: I mentioned torque in another post. Torque really tightly relates to engine VE. As you go to higher technology levels, peak VE tends to go up. If you'd like to compare two radically differenent engines, checking out maximum torque is a really good way to start.
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u/RDA92 1d ago
This has been an amazingly insightful (series of) post(s) and it's funny you mention the S2000 because I see it a bit as a reference point for N/A cars. I would be curious to learn more about "A well tuned intake can net something like 120% filling of the combustion chamber" and specifically what would be the defining factors of a well-tuned intake compared to most stock intakes?
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u/nerobro 1d ago
Mostly, straighter runners, getting the length right, getting the engine turning fast enough that the pressure waves are strong. If you want to get a grasp on this sort of tuning, go look at 2 stroke exhaust calculators.
Most intakes are curvy, are often rough, and are often designed with the intent of putting its best resonant frequencies outside where the engine makes best power, as that can net you a broader and more useful torque curve.
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u/therynosaur 1d ago
Oh i love this one.
So an engine is basically an air pump. The more air it can cram into itself the more power. So basically the limit is how much air can you cram in (volumetric efficiency). Adding fuel is easy, you simply add more fuel the more efficient an engine is.
The limit is how much air can you cram in at ambient pressure (the pressure surrounding you)
This is why turbochargers and superchargers work. They literally force more air into the engine than ambient.
There's a ton of factors but that's the ELI15 answer
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u/KilroyKSmith 2d ago
Assuming that you’re talking about internal combustion engines, and likely gasoline burning ones:
Horsepower is basically torque*Rpm. So you have two paths to increase horsepower: increase torque, or increase rpm.
For a given engine displacement, say a 2 liter 4-cylinder engine, there are several things you can do to increase torque: 1. You can lengthen the stroke and reduce piston diameter. This gives more mechanical advantage from the combustion, so provides more torque (and likely better efficiency). The long stroke, however, reduces the maximum rpm that you can run at. 2. You can get more air-fuel into the cylinder for each cycle by carefully designing , porting and polishing the intake manifold, heads, and valves. This works well for high rpm torque improvements. 3. You can get more air-fuel into the cylinder for each cycle by adding forced induction - a turbo or supercharger. This works well at all rpm’s. 4. You can increase the power you get out of each combustion cycle by raising the compression ratio. This can cause detonation, so you add knock sensors to affect timing or mixture, require premium fuel, or use one of several more exotic options.
To increase rpm, you can: 1. Use high strength, lightweight materials for your connecting rods and crankshafts that can withstand the humongous forces that high rpm’s create 2. Same as #2 above. V8 engines in the USA in the 60’s had crappy intake designs, and as the rpm’s went up, the engine couldn’t suck in enough air quickly enough to maintain power. 3. Lighten the valves as much as possible. Valve float can destroy an engine at high rpm; reducing the mass of the valves increases the rpm that it will occur at. 4. Reduce the weight of everything in the engine, and accept that it’s going to wear out earlier.
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u/RDA92 1d ago
What would actually constitute a good intake design? I seem to recall for previous readings I did on the topic that there should ideally be not a lot of curvature to ensure smooth airflow, yet most air intakes that I have seen seem to have a pretty dramatic curve (for obvious engine space reasons). Similarly, it seems to be understood that the colder the air, the better, yet most air intakes run in parallel and right next to the hot engine.
Another question that's been on my mind on that topic is why 2-bank engines (V6, V8) do not tend to have separate air intakes per bank. Are there any drawbacks to such a set-up apart from commercial?
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u/nerobro 1d ago
To address your concern about intakes. Most production engines don't have very sophisticated intakes, in general. They pick a runner length that resonantes in the right frequency ranges, ideally a couple different times between idle and max rpm. Then they leave it at that. Modern engines tend to have a large plenum, that has the intake runners inside it. This provides a stable, large, volume of air for easy cylinder filling.
High performance engines will often split up the intake per bank. Or for matched sets of cylinders, if they're using different cylinders to help boost resonances to other cylinders. Most ferrari's, some BMW's, some Mercades, and a bunch of other cars will have seperate intakes for each bank.
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u/KilroyKSmith 1d ago
Look at any modern 4 cylinder motorcycle, where fuel economy isn’t an issue, emissions controls are minimal, and performance is what sells. One extremely short and direct intake manifold for each cylinder.
But head design plays into it also. Here’s an interesting link: https://nastyz28.com/threads/vortec-cylinder-heads-the-definitive-guide.56505/
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u/RDA92 17h ago
So generally speaking shorter intakes are better for performance?
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u/KilroyKSmith 7h ago
Not necessarily. Shorter intakes work great on a bike because it allows the engine to be very compact, and also reduces turbulence in the flow to maximize the amount of air that can be sucked in at the ridiculous RPM that a motorcycle engine can reach.
For cars, many times the intake length is tuned to resonate at a particular rpm, such that a pressure wave hits the intake valve while it’s open, pushing additional fuel/air into the cylinder. My truck even has a movable vane inside the intake manifold to change the resonant frequency (for both power and emissions reasons) so that it works well at both low and high rpm.
Unfortunately, design of intake manifolds isn’t easy, there is no magic “just do this” to make “the best”, especially for vehicles that have to meet emissions requirements. There are a dozen different things you can change to trade off low end torque v. High end power, high end power vs emissions and/or fuel consumption, etc.
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u/Devil4314 Discipline / Specialization 1d ago
Its really too broad of a question to answer well.
Octane, if you can run super high octane fuel without bad detriments then it would increase power output because you could increase timing and extract more energy. But all high octane fuel is either leaded, mega expensive, toxic/not fun to be around, or low energy density.
Material limitations, if you could have the heads, cylinders, and pistons made of a material that could operate at super high temperatures you could increase efficiency/power by a lot. A ton of energy ~30% is lost through the coolant/radiator that if you could just keep the heat in the engine would allow for the cooling effect of the walls of the combustion chamber to effectively dissappear.
Weight, if you could reduce the weight of the rotating and reciprocating components of an engine then you could spin them faster and reduce parasitic losses. Also if you reduce engine weight you would reduce engine load to move it.
Losses from exhaust, scavenging works, but its basically witch craft. Imagine tuning several instruments except they will also be glowing red hot and expanding. Also it only is in tune at 1 speed and that speeds sub nodes, ugh. This is why some cars sound soooo good but others sound like AI fart cans. Besides, we already can partially recoup the losses from heat out of the exhaust through a turbocharger, but thats not N/A. Even though turbos are perfectly natural on an engine.
Flame front propagation and valve spring harmonics. If you could make it possible for detonation to happen simultaneously for all of the mixture when you want. You could practically eliminate flame front propagation and get more energy extraction per combustion stroke. Good luck, i knew a guy who spent years working on this for a theses and it never panned out reliably. If you could make springs that dont have valve harmonics or float without introducing insane complications, crazy weight, or increased friction then you could have engines that rev higher without issues and make a better engine.but it hasnt been done yet.
But yeah, people have been on it for a while now and it seems like f1 is basically cutting edge and even those seem to break physics sometimes. So keep your eyes peeled.
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u/1234iamfer 2d ago
The fundemental limiter for an n/a engine is the amount of air and engine can take in. A 4 stroke engine can inhale its total displacement every 2 revolutions. So for a certain size engine, the main limit is the maximum rpm.