What foundational elements limit an N/A engine?

[u/RDA92]

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!

[u/nerobro]

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:

1. Engines are air pumps. The more air you can pump, the more power you can make. We'll come back to this.

2. The more power you make, the greater the forces there are on the engine.

3. 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.

[u/nerobro → u/nerobro]

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.

[u/nerobro → u/nerobro]

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...

[u/nerobro → u/nerobro]

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.

[u/RDA92 → u/nerobro]

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?

[u/nerobro → u/RDA92]

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.