Do Turbo Engines have more cylinder pressure than NA engines?

Submitted by walmartart t3_10fq8s1 in askscience

Background:

Friend says turbo engines (typically) run lower compression ratio than naturally aspirated engines, and concluded turbocharged engines don't require as high of octane as an NA engine of the same compression ratio.

From my experience with ECU tuning, I know that maximum Manifold Absoulute pressure is often (roughly) double in turbo engines compared to NA (NA is retricted to atmospheric pressure: 1ATM/~14.7psi/~1013mbar)

Question:

Assume all variables & designs (power, fuel economy, displacement) are the same EXCEPT for compression ratio, peak MA pressure (MAP), and means of induction.

Does a turbocharged engine produce higher cylinder pressure than a naturally aspirated engine?

Additional question:

Assume the same scenario as above, but now compare at an equivalent MA pressure. Say both engines cylinder pressure is measured when both engines are at 1ATM (maximum MAP for NA, but typically half maximum MAP for a turbo engine)

In this scenario, which engine will have a higher cylinder pressure?

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Comments

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Andis-x t1_j4yppqv wrote

Your friend is right and then wrong.

Yes, turbocharged engines usually have lower static compression ratio. For example my Subaru had 8.5:1. While NA engine is typically 11 to 13. But why ?

To compensate for higher air pressure from turbocharger. To make more force you need more pressure that comes from explosion. By forcing more air in, you can add more fuel and generate more pressure.

Turbocharged engines tuned for performance need better octane fuel, because you are trying to compress air that is already compressed, meaning that temperature raises more quickly.

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Yen1969 t1_j4yw2yc wrote

The primary reason for lower compression ratios on engines with forced induction is that the compression process (from turbocharger or supercharger) heats up the air, and even with an intercooler, the increased temperatures raise the risk of pre-ignition (or detonation, or knock). Higher octane increases the auto ignition temperature, allowing for greater pressures in the same engine without knocking. The engine static compression ratio can also be lowered to reduce how much the air will be heated by the compression stroke.

But this is actually not necessary. It is just an extra safety margin that is favored by people who really really don't want to take more risk than necessary. For example there is a turbocharger kit for the Porsche 11:1 CR flat 6. It is limited to 4psi, but it still provides a rather large increase in power, because you will make more power for a given boost pressure with a higher CR.

However... all of this is largely beside the point of your question. There is an easy way to shortcut all of the nuances of the variables when you are looking for cylinder pressures: Torque. More cylinder pressure = more torque on the crankshaft. So the way to get the answer to your question is to examine different engines with different CRs, different boost pressures, etc... And just look at what makes more torque.

I have had the same 1.8L engine boosted, and dynoed, at pressures from 5psi to 15psi, and with CRs of 10.5:1, 9.5:1, and 8.6:1. (and have had other non-boosted versions of the same car/engine) All on 93 octane, so that was a controlled variable. I could run more boost on track without knock with the 8.6:1, but the torque/power peaks didn't actually vary much. Naturally Aspirated doesn't change this. Cylinder pressure is directly related to torque.

Anecdotally, I vastly preferred lower boost and higher CR for a given power level, since the turbo reacted quicker from a higher off-boost mass going through the turbine, and when spooling it gets to it's lower peak boost much faster. The end result is that the car had much better partial and transient throttle dynamics, which made road course "racing" (non competitive) vastly better

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walmartart OP t1_j4z6b4x wrote

Very informative response, thank you for your time and anecdotes

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SatansCouncil t1_j53i4wx wrote

High compression ratio and lower boost makes sense for roadcoursing an overbore, high revving engine. Superior part-throttle reponse at the cost of max power.

But imo, better streetable power can be had with a stroker with a comfortable compression ratio and a bigger turbo/intercooler that has a decent island of efficiency. Superior power at the cost of part throttle capability.

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