NM of diesel and benzine

[SilenCe]

Why have cars with diesel engine more NM than cars with benzine eninge although they both have the same horsepower?

[martin100]

Don´t exacltly know. I suppose because all modern diesel engines got a turbo and most "normal" engines don´t :roll:

[Vansquish]

It has to do with the way the fuel is burned actually... You can have a not TDi engine and still produce a hell of a lot more torque than a similarly sized gasoline engine. If you google-search I imagine you'll find a more precise answer than I can give hehe.

[Ian_yamaue]

Exact, the diesel generates more pressure as burn than gas.

[Vansquish]

That is an extremely limited view of the situation. If you want to look at the amount of energy released during combustion, my guess is that Gasoline actually carries more BTUs per unit volume than does Diesel. However, the amount of torque supplied by the diesel engine may have more to do with the manner in which it is burned. It is a compression-ignition engine as opposed to the gasoline internal combustion engine. The compression ratio for a diesel is extremely high compared to a gasoline engine.

[mindgam3]

Diesels have a bigger bore and smaller stroke. That's also why they run that low rpm's...

smaller stroke usually gives higher revs as the piston has to move a smaller distance for every revolution

[Vansquish]

I was too lazy to do this before, but this has to stop! I googled and found this in about 30 seconds:

Diesel fuel has about 11 percent more energy per gallon than gasoline too. And if all of that isn’t enough, a diesel is also more efficient than a gas engine. We could leave it at that, but the idea here is to dig a little deeper into these things to give you the knowledge to understand and build on for future discussions. With that in mind, let’s take a closer look at diesel stroke length and cylinder pressure.

The old adage that a long stroke is good for torque is true. The longer the stroke, the more offset the crankshaft pin has 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. The Merriam-Webster dictionary defines torque as: “a force that produces or tends to produce rotation or torsion…; a measure of the effectiveness of such a force that consists of the product of the force and the perpendicular distance from the centerline of action of the force to the axis of rotation.” That’s a mouthful, but the part about “the product of the force and the perpendicular distance from the centerline of action of the force to the axis of rotation” is critical to our understanding of generation of torque in an engine. The greater the crankpin offset to the centerline of the crank, the greater this perpendicular distance will be for any degree of crankshaft rotation after top dead center (TDC) and before bottom dead center (BDC). Consequently, the more leverage the pressure on the piston top that produces or tends to produce rotation of the crankshaft, or the more force it can exert: torque.

Diesels are usually designed as long-stroke engines specifically to generate torque. This is possible because the heavy components necessary to withstand the high compression ratio, and high cylinder pressure on the power stroke, of a diesel limit engine speed anyway, so excessive piston speed associated with long strokes isn’t a problem. Some heavy-duty truck diesels operate at a maximum of only 2200 RPM. Lighter-duty truck diesels may redline at 3000 to 3500 RPM, and 4000 RPM is considered a “high-speed” diesel. Let’s just summarize by saying that diesels are usually designed with as long a stroke as practical for the desired peak engine speed. For example, the 5.9L Cummins in Project Sidewinder has a stroke of 4.72 inches and a bore of only 4.02 inches. By contrast, most gasoline automotive engines have a stroke length that is shorter than the bore diameter.

There’s a negative side to increasing stroke other than being an RPM limiting factor. The longer the stroke, the greater distance the piston must move during each stroke. At a given RPM, that means the piston has to travel faster (higher average velocity) to cover that distance than it would if the stroke was shorter. Now remembering that the piston is essentially stopped at both TDC and BDC, to achieve that higher average velocity during the stroke, the piston must accelerate faster during the first half of the stroke and decelerate faster during the second half. This increased acceleration and deceleration takes energy – lots of it. Fortunately, the negative torque of accelerating the piston is largely balanced by the positive torque of the deceleration, but the loads on the crankshaft, piston, the piston pin, connecting rod, and rod bearing during all four strokes of a four-cycle engine increase dramatically with increases in stroke (or piston speed).

Now let’s discuss effective cylinder pressure on the power stroke of a diesel as compared to that in a gasoline engine. We’ve already mentioned that higher turbocharger boost raises the effective cylinder pressure, but let’s look at what else comes into play. In “Understanding Today’s Diesel” elsewhere on this site, the way fuel is introduced into the cylinder is thoroughly discussed for both gasoline and diesel engines. Gasoline engines mix the fuel with the air before it enters the cylinder, so when the intake valve closes, the power potential of that air and fuel charge is set. The timed spark ignites the mixture and cylinder pressure rises to a peak at roughly 15º after TDC. Because the combustion process takes time, combustion may or may not be complete by 15º after TDC depending on engine RPM, but for all practical purposes, we can say that the process of combustion is concluded early in the power stroke and that no more heating of the working fluid (the gases in the cylinder) occurs. This means the force acting on the piston top is highest at a time when the connecting rod has very little leverage on the crankshaft pin. As the crankshaft continues to rotate past TDC, the leverage the piston can exert increases, but the pressure on the piston top is dropping quickly. This, too, is discussed in the aforementioned article.

Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. This means the average effective cylinder pressure acting on the piston is higher in a diesel than in a comparably sized gasoline engine. The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels. And all of this taken together makes it apparent why diesel engines have to be built with such robust parts to withstand this high cylinder pressure and torque.

Of course, injecting fuel beyond a certain point on the power stroke of a diesel does no good because there isn’t time for combustion to conclude before the exhaust valve opens near BDC.

Hope that clears things up a bit. It's from this webpage http://www.bankspower.com/Tech_somuchtorque.cfm

[SilenCe]

Thanks man

[Vansquish]

No prob.

[Wutputt]

I agree completely with Vanquish long post but I have a few remarks about some other posts.

Why have cars with diesel engine more NM than cars with benzine eninge although they both have the same horsepower?
They don't have both the same horsepower if you compare them correctly. A 2.0 turbo petrol engine produces far more horsepower than a 2.0 turbodiesel engine, since it can rev much higher due to lighter moving parts (like pistons, connection rods, etc) as explained in Vanquish post. Don't mix a turbodiesel up with a NA petrol engine.



Diesels have a bigger bore and smaller stroke. That's also why they run that low rpm's...

smaller stroke usually gives higher revs as the piston has to move a smaller distance for every revolution
Idd, a smaller stroke gives less acceleration and so less inertial forces. Which means the engine can rev higher. Also lighter materials reduce the inertial forces. But as explained in Vanquish’s post, the main reason why diesel engines run at lower rpm, besides the slighter bigger stroke, is due to the fact that the moving parts have to be more robust due to the higher pressure.