iolithblue

i drove a c320cdi (w204) for about 1000km's the other week, and came away deeply impressed. the only one question i had was.... why wont a diesel rev past 4500rpm????

Untertürkheim

Diesel engines have much higher compression ratios than gasoline engines, that is the most fundamental why not. Then again, they produce so much more torque than a comparable gasoline engine, one does not need to rev higher than that.

lkchris

The V12 Audis that have won LeMans 24-hr the past couple years reportedly redline at 5500.

harkgar

Ken,

You are talking about a racing diesel engine though. The old standard Toyota MR-2 four cylinder DOHC engines can rev up to 6,000 rpm with ease, like a motorcycle engine. The 2008 Ford Powerstroke V-8 starts getting its maximum torque at 2,000 rpm, all 650 pound foot of it.

I think it is fair to say that road going diesel engines do not rev as high as their gasoline counterparts.

Diesel Benz

Our diesel (all are diesel of course) trucks rev hardly a maximum 2000 rpm and work best at about 1300 rpm (660 hp engines on pretty typical commercial trucks). A motorcycle would rev easily above 10 000 rpm, I'm not familiar with these but this one has max power at 12500 rpm.
http://www.yamaha-motor.co.uk/produc...view=techspecs

Old pre-chamber diesels actually revved higher than current direct injection engines. I'm not an expert on engines but the low maximum rpm should mainly come from the fact that fuel is injected during the combustion stroke, not "at once" before the air is compressed (like a gasser with indirect, or even direct injection) and the diesel fuel simply does not burn quickly enough to allow higher rpm.

I've seen forum members who are really extreme experts on engine technology, searching earlier posts would give a lot of detailed info (this and other forums).

240D 3.0T

Mercedes' diesel won't rev past 4500rpm because that is how high they designed it.

It's about torque, stroke length and piston velocity. The longer the stroke, the more torque the engine makes and the higher the piston velocity is compared to crank rpm.

Mercedes only makes their engines rev to 4500rpm because they designed them with a long stroke to give them massive torque. Compare that to a Honda S2000 that revs to 10,000rpm and has the torque of a lawn mower.

DESAL

I drive a 05 E320 CDI and when that turbo kicks in, who cares about torque

Read this review on the 320;

In the E320 the six-pack plant produces 201 hp at 4200 rpm plus substantial torque of 369 lb-ft available at only 1800 rpm.

It's rated to earn fuel economy scores well above 30 mpg for highway cruising and, when tied to a fuel tank of 21.1 gallons, puts the cruising range out at 700 miles or more -- which far exceeds any passenger's cruising range without inserting a rest stop.

Track times indicate the E320 CDI can accelerate from a standing start to the mark of 60 mph in 6.6 seconds, while the top speed, limited by electronics, is 130 mph.

And E320 CDI runs so quietly and provides such juicy acceleration, particularly in lower gears, that a driver would never suspect there's a diesel engine aboard without checking the label.

240D 3.0T

Torque IS what happens when that turbo kicks in.

iolithblue

ok, so the relativlely low rev limit on a diesel is to allow the fuel time to burn, and this is why power drops off after that?

i love the diesels, and have driven many of them, i just wanted to know why they wont rev.

scottybdiving

Achieving max HP and torque at lower RPM's also contributes to a diesel's greater longevity.

khaug

I loved your post. "Deeply impressed" very nicely characterizes my reaction to our E320 CDI after 3 years, which led to my purchasing a VW Touareg V10 TDI this spring. The thrust of diesel torque has to be experienced to be believed!

In street driving, I seldom find it necessary to use more than 3900 rpm or so in our diesels. By contrast, our Porsche GT3 is dull below 4,000 and then will pull very strongly to the 8400 rpm redline.

This illustrates two VERY different power/torque curves. A friend commented that the torque curve on the E320 CDI is "backwards" - i.e., peaks at very low crankshaft speeds and falls off quickly, as compared to most gassers. That's just fine with me, and makes for relaxed street driving and towing. The torque thrust (553 lb/ft in our Touareg @2K rpm) is just unbelievable. For towing, i.e., at highway speeds with the turbos spooled up, the Touareg is simply unsurpassed! It will literally climb mountains (WVA turnpike) in 6th gear while towing the GT3 in its trailer. But it won't pull 154 mph at the end of VIR's back straight like the GT3!

2JZ

There is also a "time" issue when trying to get a diesel engine to rev to high rpms. The Audi and Peugeot teams could not spin their race engines very high because it takes a "relatively" long time to build the astronomical compression ratios needed for efficient combustion in a diesel application.

240D 3.0T

No, Audi and Peugeot don't revv high because that would mean a short stroke and loosing the diesel's torque and fuel economy advantage.

dieselfan

Time does have something to do with it. This is a compression ignition design (Diesel) and diesel burns slower then gasoline. You have to get the burn cycle complete (time) and exhausted from the combustion chamber and it has proven to work better at lower rpm's. The manufacturers make diesel motors long stroke to take advantage of the slow burn and torque associated with both of these things.
dieselfan