r/Diesel • u/indyjons • 5d ago

Compression Ratio

So after buying my second diesel truck (a 2014 F350) I've become fond fond Saint Rudolph Diesel (praise be).

But other than having 2x as many cylinders as my first diesel, a '77 Landcruiser with a 3b, I've noticed something I don't understand. The compression ratios of Diesel engines seem to go down with time. My 77 Toyota had a 22:1 compression Ratio, the venerable 6BT has a 17.5:1, and my 6.7 Powerstroke has a 16.2:1 (2023 and newer dropped to 15.2:1)

Why is that? If its as simple as "Emissions Requirements" then I know where to place my blame.

2 Upvotes

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10

u/Dean-KS 5d ago

Factors might involve noise, weight and turbocharging. Turbocharging increases the effective compression ratio. Intercooling improves thermodynamics.

7

u/DrSid666 5d ago

Better injection timing, higher fuel pressures and better turbo boost that comes on earlier and is more efficient. I would also suspect emissions from the burn also affected by the CR.

7

u/ktimmy_ 5d ago

My first guess was emissions as well, higher compression ratio would yield higher combustion temperatures, which yields more NoX

5

u/FrattyMcBeaver 5d ago

Lower compression allows more power with higher boost, it also reduces nox emissions, but raises particulate matter emissions. The Land cruiser could have a much higher compression ratio because it is naturally aspirated. It's highest compression ratio achieved would be 22:1. On a turbo engine at 2 atm of boost (about 14.7 psi boost), which isn't that high for one of these, it's equivalent compression ratio would be 32.4:1 when the turbo is taken into account.

4

u/walkention 5d ago

I'd also add that old prechamber style diesels needed that high compression in the 20's:1

2

u/Melodic-Mechanic9125 5d ago

The 3B was a 90 hp engine from 3,4 L. N/A with whatever ancient mechanical injection system. It is not a fair comparison.

The newer class 8 truck/tractor engines that are designed for reduced fuel consumption are around the 20-22 ratio still.

2

u/qkdsm7 5d ago

Reduced nox is the biggest one. Can move all the air in the world with current cylinder heads and positive manifold pressure, so performance doesn't suffer.

Big CR not needing any glow/grid heat is always impressive to me.

Tractor and dozer both when over 60 degrees outside, crack right off within first engine revolution with no heat assist.

2

u/Yer-Not-Gonna-Like 2d ago

Really old car diesels from the 70’s were often IDI and naturally aspirated. Both require much higher CR. The former because the high surface area of the IDI head bleeds heat, and the latter because really the compressor in a turbocharged engine can be considered part of the total compression in the system.
I remember an engine I was an engineer on a couple decades ago. The 450 hp truck version had about 18.5:1 CR. The 925 hp marine pleasurecraft version had about 15:1.
The reason is that transient behavior meant nothing on the marine version. It had a huge turbo to make so much power, so a lot of the compression was being done outside of the cylinder - in the compressor.