Question for the experts on exhaust dynamics

[texlurch]

I am trying to determine how the exhaust of a diesel compares to that of a gas engine, given equal displacements.

In other words, a diesel exhaust is 150% of a gas engine, for a given rpm. I figure it would be more at lower rpms, but just a rough estimate.

What I am trying to determine is how the turbine side of a given turbo will act on a diesel versus a gas engine.

I know there is more coming out of the oil burner, you have close to twice the compression ratio, plus diesel releases more heat from combustion.

I also have a chart off IHI's website; it lists the same model turbo for diesel or gas burners, looks like the displacement is around a 1.6 factor, gas to diesel, so 150% should be close?

[slow4dr]

HMMMM?

Given equal parameters they would both flow the same amount of air. The massive compression ratios is why a diesel will spool a larger turbo. ;)

[texlurch]

So you are saying the exhaust flow on the gas and diesel motor would be the same? Seems strange to me.. I know for equal displacement the intake side airflow would be similar, but I would have bet that there would be a pretty good difference on the exhaust side. Especially with the compression ratio diffference, and the longer burn time for the diesel.

Here is where I am trying to get. If a 6.5 diesel hits max boost and rpm on the turbo at 3000-3500 rpm, what would happen with the same motor on a 6.5 gas engine? I am saying it would have to run at a higher rpm to spin the turbo at the same rpm, based off exhaust flow. My guestimate is around 5000 or so on the gas motor, to get the same pressure and flow out of the turbo. At the same time, much over that and you would overspin the turbo and go past it's efficiency range.

Close?

[slow4dr]

So you are saying the exhaust flow on the gas and diesel motor would be the same? Seems strange to me.. I know for equal displacement the intake side airflow would be similar, but I would have bet that there would be a pretty good difference on the exhaust side. Especially with the compression ratio diffference, and the longer burn time for the diesel.

Here is where I am trying to get. If a 6.5 diesel hits max boost and rpm on the turbo at 3000-3500 rpm, what would happen with the same motor on a 6.5 gas engine? I am saying it would have to run at a higher rpm to spin the turbo at the same rpm, based off exhaust flow. My guestimate is around 5000 or so on the gas motor, to get the same pressure and flow out of the turbo. At the same time, much over that and you would overspin the turbo and go past it's efficiency range.

Close?

On a gas engine the exhaust gases are expanding more as it exits the chamber which will spool the turbo up quicker.

The higher compression ratio is what gives the diesel the advantage.

[jlm44]

The key here is expanding gases not flow, turbos work off of heat expansion, thus giving the diesel the advantage due to higher compression = more heat present.

[slow4dr]

The key here is expanding gases not flow, turbos work off of heat expansion, thus giving the diesel the advantage due to higher compression = more heat present.

Gasoline burns hotter/diesel burns slower. Typical max EGT's in a diesel exhaust is 1400 degrees where as a gas will be as high as 1800.

Yes, more compression will produce more heat. Comparing all things equally the gas engine will have the advantage.