View Single Post
Old 12-03-2019, 10:56 AM   #7 (permalink)
Master EcoModder
Ecky's Avatar
Join Date: Dec 2011
Location: Vermont
Posts: 4,005

Gaptooth (retired) - '00 Honda Insight
Team Honda
Gen-1 Insights
90 day: 54.26 mpg (US)

Such Fit - '07 Honda Fit Sport
90 day: 41.27 mpg (US)

Connect - '15 Ford Transit Connect XL
90 day: 24.03 mpg (US)

K-sight - '00 Honda K-sight
Team Honda
90 day: 46.38 mpg (US)
Thanks: 1,774
Thanked 1,695 Times in 1,084 Posts
A more nuanced explanation I found:

Turbos have an effective range where they move the most amount of air with the smallest heat generation. The translation to how many PSI that is depends on the size of the engine.

Now, how does this help to make power? It's a combination of putting more oxygen in the cylinder, keeping the intake charge cool [so that you can go back to the first item of having more oxygen in the cylinder] , and most importantly, raising the peak cylinder pressures during compression.

So far, I haven't mentioned compression ratio at all [current case exlcuded], and that's because for the most part, the info so far on this thread is accurate. So why, then, do most people view a lower compression engine better for boost?

The answer is actually pretty simple. You can only increase the cylinder pressures during compression so much before gasoline spontaneously ignites [detonation, pinging, whatever you want to call it, it's bad]. What happens, then, is that when you boost a high compression engine, the boost makes a larger effect on the peak cylinder pressures than the same boost level on a similar engine with a lower compression ratio. So why, again, do most people view a lower compression engine better for boost?

With a lower compression ratio to deal with, you can inch yourself closer to the maximum cylinder pressure for the given fuel you're using easier than you can with the relatively constrictive range of boost you can deal with on a high compression engine. The downside, again, is that you need a turbo that will operate efficiently to provide the necessary amount of air according to the three guidelines I covered earlier.

Another thing to keep in mind is that anytime you compress air, it gets heated, and since you make your power from the rapid expansion of air in the cylinder, the lower the beginning temperature theorhetically yeilds the greatest change in temperature in the cylinder, which means the fastest/most forceful expansion, which equals power. I take this to mean that if you can compress more air more efficiently and then compress it in the cylinder with the least amount of extra heat generated, the more power you will make. So far, most turbo engine manufacturers/racers agree with this.

In this forum, pretty much anyone that mentions CFM talking about a turbo is likely to be someone that I've told/taught about boost. CFM [cubic feet per minute] is a measure of how much air the compressor will flow efficiently [ie- low temperature increase], and so that is the context it is mentioned, not as a literal definition of how much air the engine will ingest in a cycle, only how much air it can move effectively enough to be useful.

I'm no expert. Some of what I say is extrapolation.

You can drop peak cylinder pressure by retarding timing, all else being equal. You can thus maintain the same peak with increased compression by retarding timing - start combustion later and rising cylinder pressure will happen later, peak pressure later in the stroke after peak compression so your total peak cylinder pressure stays the same but happens at a different stroke angle. That peak cylinder pressure is your ceiling with a given octane of fuel. Dumping fuel (really rich AFRs like 10:1) can help by cooling the combustion chamber but that's also a loss.

What I'm uncertain about is if you can remain neutral at WOT with extremely precise engine tuning, or always have a net loss with high compression + boost. That is to say, after a certain point you're losing power and efficiency because you have to start combustion so late (to keep peak pressure from getting too high) that more and more of the combustion energy is just being flushed out the exhaust and never pushes on the piston, but is this equal to or greater than the gains from increased compression? This applies even without boost; more compression is not always better, but it may or may not always be worse.

I'm aware that engine tuners often use exhaust gas temperature (per cylinder) to find ideal timing advance. Advancing it to a certain point drops exhaust temperature because more of it is turning into mechanical energy. Retarding it and watching exhaust gas temps can help find the minimum timing needed to make power with high octane fuels like alcohol, which is typically not knock-limited.

Ideally at a given compression ratio you would find where advancing your timing more provides no more power. If you can do this on the fuel you're using without detonation, you have room to add a little boost OR increase compression, with no negative consequences.

Last edited by Ecky; 12-03-2019 at 01:23 PM..
  Reply With Quote