Diesel torque and sound - PeachParts Mercedes-Benz Forum

kerry · #1 12-12-2003, 01:18 PM

Here are a couple of posts from theSprinter list at Yahoo explaining diesel torque.

Oh, and one reason a diesel has so much torque is because the fuel burning cycle is
> much, much longer than on a gasser. Instead of a single combustion event when the
> plug fires in a gasser, the fuel in a diesel keeps burning as it's being injected -- the
> added torque is because the fuel keeps burning (and pushing the piston down)
> through more of the power stroke. The sound of a diesel is the multiple shock waves
> colliding in the combustion chamber as the fuel is injected. The reason newer diesels
> are quieter is because the current generation of electronic fuel injection is much
> better at controling fuel flow and a lot of fuel can be quickly injected thus reducing
> noise.
>
> And while we're at it -- some folks think the limited RPM range of a diesel is due to
> it's greater rotating mass. While that plays a part, the main reason is the slowness of
> diesel combustion -- if the engine is turning too fast you can't burn all the fuel. Many
> diesel engines in commercial trucking have 2 rpm ranges -- one while under power
> and the other while in "jake" braking mode. In the latter fuel burn rates aren't an issue
> (because fueling is turned off the the valve action changed to create an air
> compressor in the engine) and the engine is allowed to turn faster to provide more
> braking effect.
>
> -Mike
>

There are many factors that come in to play regarding torque output in a
diesel engine. As mentioned, the combustion phase is of longer duration
relative to to the cycle of the piston on the downstroke. Overall, the
mean effective pressure in the cylinder of a diesel engine is much
higher. In addition, diesel engines, in comparison to gasoline engines,
generally have a longer stroke. This is needed to accomplish higher
compression ratios. The longer the stroke, the more offset the
crankshaft pin is from the centerline of the crankshaft. This means the
connecting rod can exert more leverage to turn the crank as the piston
descends on the power stroke. Torque is directly related to the force
on the piston and the length of the moment arm, the crankpin offset
in this case. In addition, a gasoline engine develops its highest
cylinder pressure much closer to TDC, top dead center. At this point,
the moment arm is the shortest with respect to the connecting rod
and crank pin. In reality, a gasoline engine develops its highest
pressure some point after TDC, but is still at a much less optimal
point relative to the crank arm position to develop the most torque.

--

Ray Muno http://www.aem.umn.edu/people/staff/muno
University of Minnesota e-mail: muno@aem.umn.edu

JenTay · #2 12-12-2003, 01:41 PM

too deep for me. i'll take your word for it

psfred · #3 12-12-2003, 02:37 PM

The low rpm limit most people are familiar with on diesels is a result of the usual American approach with very long stroke and fixed injection timing. The old Mack and Detriot 2-strokes were the worst -- no power below 1800 and none above 2100 rpm, and it threw a rod at 2200. Modern engines tend to run a lot faster, especially the European designs (my Volvo redlines at 5300, the 61x MB engines at about 5000, although you CAN push the turbos higher at the expense of engine life, the 60x engines run more like 4200). The newer American designs are considerably more flexible, too.

The fixed injection timing/long stroke combination works very well for a single speed engine, as in running a generator, but variable injection timing and short stroke (like MB and Volvo diesels) makes for a much more efficent engine over a much larger rpm range.

The noise is the result of late ignition compounded on fixed timing engines by the fact that the injection it much too early at idle -- injection timing is set for high rpm only.

Peter

bjcsc · #4 12-12-2003, 04:41 PM

Nice information, however it is somewhat misleading as I think it could lead people to think that a diesel compared to a gas engine of the exact same size would have higher torque and hp., which of course is not true. The diesel would have significantly less torque and hp...

psfred · #5 12-12-2003, 08:48 PM

Generally, same or higher torque, less hp.

My 2.4 L Volvo engine makes 145 ft/lbs torque, 105 hp. My 300D makes 149 hp and 195 ft/lbs torque, more hp AND torque per cubic inch than a Chevy Vortec V6! Newer engines are even better (gas ones are, too, for that matter, though!)

For any given hp rating, the diesel will use about 1/3 less fuel.

Part of the noise from a diesel is in fact sound generated by the fuel injection. Turns out that there is a supersonic shock wave formes right behind the flame front. Knowing this has helped to make the newest diesels more efficient, less smoky, and quieter.

Peter

Gurkha · #6 12-12-2003, 10:16 PM

The HINO 4009cc four cylinder diesel develops 120bhp at 1600rpm and develops 315lb of torque at a staggering 1300rpm and all this for a mesly four cylinder non turbo, put a turbo in it or upgrade to the six and see the firues go up. All this from a pushrod operated engine, their newer OHC J series rev higher and develop even more.

The point is that diesels are the most forgiving luggers around and for trails in a 4WD nothing comes closer to them, not to mention their inherent durability.

Also consider this, from its introduction in the Indian market, the OM 616 has been primarily used for mini truck purposes, a statement grossly miss used here, a truck rated for 3.5 ton payload was regualarly over loaded to almost 5 tons and even with bad maintainance these engines would stretch to over 200,000 KM. Show me one 2.4 liter gasoline which can do this and last even 30,000 KM. Today the OM 617 is used for hauling 8 ton payload and the both the OM616/671 is used for tractors. Diesels are venerable to say the least.

bjcsc · #7 12-12-2003, 10:17 PM

Quote:

Originally posted by psfred
Generally, same or higher torque, less hp.

My 2.4 L Volvo engine makes 145 ft/lbs torque, 105 hp. My 300D makes 149 hp and 195 ft/lbs torque, more hp AND torque per cubic inch than a Chevy Vortec V6! Newer engines are even better (gas ones are, too, for that matter, though!)
Peter

Not to beat this (or you) to death as I love my diesel and prefer it to gas, but a stock Mercedes 617 makes 120bhp @ 4350rpm and 170 ft/lbs torque, but Chevy Vortec V6 makes 180bhp @ 4400rpm and 245 ft/lbs torque. (specs for both from manufacturer) However, per cubic liter they are very close: 40bhp/L for our pride and joys, 41.8bhp for the Chevy. 56.7ft/lbs / L for us, 57.0 ft/lbs for Chevy. I'm impressd how close they are. Man the 617 is an asswhooper! Oops...can we say asswhooper??

Now, for a more direct comparison, and we'll ignore the diesel is a turbo:

MB:
126.135 350SDL I6(603.970) : 134bhp
126.024 300SE I6(103.981) : 177bhp

Chevy:
1987 LO5 V8 TBI: 210hp, 300ft/lbs
1987 LH4 V8 Diesel: 135hp, 240ft/lbs

Nonetheless, given this info and all the other advantages, everyone should drive a diesel.

OhioMercedesBoy · #8 12-13-2003, 01:57 AM

I found this video online of a 240D manual. It is in quicktime format, and has music playing in the background. It is called 240DDrive nad has the most beautiful 240D in it I have ever seen. At the start when he fires it up, the sound makes my get goosebumps... I really need my 240D bad. I drive large diesels every day in my busses, but nothing has the sound of that 240D.
~D.J.~

Gurkha · #9 12-13-2003, 07:31 AM

When it comes to sound, IMHO the OM 616/617 are one of the most docile sounding engine with very civillized noise charecteristics unlike most diesels including MB's newer CRDI ones.

Gimme a Hino DI, MB Tractor, Kamat, DAF, anyday for sound.

JerryBro · #10 12-13-2003, 08:02 AM

Horsepower is calculated from engine torque. IIRC, HP = (Torque * RPM) / 5252. IMHO what is really important is not how much HP an engine has, but where the peak is. That, compared with the peak torque value and location, can tell you a lot about how an engine will behave in a given situation.

psfred · #11 12-14-2003, 02:53 PM

Bjscs:

I was comparing the Chevy Vortec to my 603 -- it has more hp and torque per displacement than the 617.

I believe the Volvo is pretty close, too, and is an older design. Makes a lot more smoke, too.

Peter

73MB280SEL · #12 12-15-2003, 06:49 AM

I think when we compare a 617 with 120 HP and 170-ish ft-lb of torque, we are really looking at a turbo engine. A stock 617 non turbo makes something like 80 hp and 120 ft-lb of torque. These are certianly not more than a chevy V-6, which itself isn't the highest specific HP/torque engine ever produced.

I drive a 190D 2.2 daily (non-turbo) and I can definitely tell you that is NOT as fast as a wide vareity of 2.2 liter gasonline cars in the 2500 - 3000 lb category.

I think people think Diesels make more torque just because they are Diesels when in reality, it is because they are turbocharged. In fact, that is the true value of the Diesel combustion cycle from a torque standpoint: They can be turbocharged without the threat of spark knock. The amount of boost they can handle is a great deal higher than a gasser with a reasonably high compression ratio.

Sholin

DslBnz · #13 12-15-2003, 10:01 AM

Have you guys heard the S400CDI? It sounds at idle like a Cummins diesel. Seriously.

My 350 doesn't make any sound at all, except for the exhaust and a very very quiet purring from the engine.

I am not impressed with the CDI from the videos I've seen(specifically the S400). Its loud and it pukes black smoke out the exhaust just like all the others. What were they talking about when they said they were quiet and smokeless?

The C30CDI sounds almost exactly like the OM617! It has a more subdued sound at higher revs though.

73MB280SEL · #14 12-15-2003, 12:49 PM

I've heard the sprinter engine (5 cyl). I think that is a CDI, but I could be wrong. My impression was that it was pretty quiet at idle relative to an older diesel.

You know, the revolutionary part of a CDI engine is the common rail part. Direct injection diesel's have been made for some time. I wonder if anybody thought of making a common rail indirect injection diesel (CIDI). Or how 'bout a common rail conversion for a 616/617 or a 601/602/603 series of diesel. No that would be neat.

Sholin

DslBnz · #15 12-15-2003, 01:03 PM

I went to my sister's the other day and I saw a Sprinter start up. You're absolutely right. They're pretty quiet(for a diesel). But why is the V8 diesel not so?

The new CDI's use electronic fuel injection to monitor the exact pressure output of the common rail. That's supposed to help with the noise reduction. We have mechanical injection. Hmmm,.. Common rails on the old diesels? Might be a good idea there,

Seriously though, if I need performance, I'd just install an intercooler, turn the screw in back of the IP to allow more fuel, increase boost pressure from the turbo, and VOILA!

Super fast racing diesel!