Question about diesel engines

Why is it that diesel engines have so much torque and so little horsepower, and why are their operating speeds so slow compared to gasoline engines?

Diesel engines can't work at high speeds due to their slow combustion. They are therefore designed for low speeds and their higher efficiency make it possible for them to make more power at low engine speeds and since power = torque*angular velocity their torque will be higher.

Another factor is that they usually have a very high compression ratio, at least compared to what we are used to in gasoline engines. Diesel fuel requires this high compression and it does result in a great dal of combustion force on the piston.

For reasons stated by Johan the also are designed with a long stroke which means better "leverage" on the crank.

Another factor is that they usually have a very high compression ratio, at least compared to what we are used to in gasoline engines. Diesel fuel requires this high compression and it does result in a great dal of combustion force on the piston.

For reasons stated by Johan the also are designed with a long stroke which means better "leverage" on the crank.
The high compression ratio is one of the reasons for its high efficiency.

The leverage on the crank doesn't affect the torque, a long stroke must however be used to make it possible to use high compression ratios and high piston speeds for better cylinder filling at low speed. The large stroke to bore does also decrease heat losses in the cylinder.

The leverage on the crank DOES affect torque - that's why torque is measured in pound/feet (or Newton/meter) - it's a function of a certain amount of force (pound) applied at a certain distance (feet) from the rotational centerline. If this isn't clear try loosening a bolt with a short wrench, then try it with a longer wrench. That's torque; and it's a function of leverage - by definition.

There are other ways to increase compression without increasing the stroke, such as a smaller combustion chamber.

There are other ways to increase cylinder filling, such as larger valve area.

One of the best engines made for torque was the old WW-II Jeep 4-banger engines. They had the longest engine cylinders for their size to allow for a very long stroke. Very long stroke means a very long "arm" on the rod journals. Very long "arm" means more leverage on the crankshaft turning, equals more torque. That thing at idle could still drive over humps that more modern 4-wheelers had to throttle-up or stomp the gas pedal hard. However, unlike the more modern engines, the RPM range was restricted.

i'v been wonderning, because of diesels slower combustion rate. What would be the effects of a diesel operating at 9500 RPM. would the torque output fall off quicker at Higher RPMs than compaired to a gasoline engine? Or can't the injectors keep up to a pace that fast?

I know that to rev at 9500 rpm you will need a short stroke, which will reduce torque output and lower the compression ratio but compression can be corrected with a smaller bore.

To my knowledge, diesel engines are low RPM engines and have a narrow RPM range for maximum power. For example, today I drove a diesel engined truck crane that has a 15 speed transmission. For normal driving, I use the top 9 speeds. Maximum power was between 1900 to 2300 RPM. 2300 was maximum governed engine speed. That 400 RPM range is so narrow that "splitting" the gears is a necessity, otherwise the engine would lug-down. A gasoline engine has a much wider RPM range of smooth continuous power output. Listening to the diesel engines in automobiles and pickup trucks, they also sound to be low RPM engines. The main advantage of diesel engines is that one gallon of diesel fuel puts out more BTU's than one gallon of gasoline. That translates into better mileage.

One of the best engines made for torque was the old WW-II Jeep 4-banger engines. They had the longest engine cylinders for their size to allow for a very long stroke. .

Wasn't that the "Go-Devil" engine?

i'v been wonderning, because of diesels slower combustion rate. What would be the effects of a diesel operating at 9500 RPM. .

In all probability the intake & exhaust passages would restrict breathing enough to prevent that from ever happening.
A high RPM engine will have a larger cross-section to the ports for easier breathing at high rpm. Resticting the size of the port restricts the rpm. That's how a throttle works - by restricting the intake passage.

I don't know what they called the WW-II Jeep engine. The only thing I thought when preparing to sleeve one of the cylinders was, "Holy smokes...where am I going to get a sleeve that long!"

A longer crank leverage DOES NOT affect the torque from an engine, this seems to be a common missunderstanding. With a given cylinder pressure and cylindervolume you won't get any extra torque by increasing the stroke, only if it increases the volume. This since, for a given volume, if you increase the bore you will get a larger area and therefore a higher force but the radius the force act on is smaller so the torque will be constant. This will also be the situation if the stroke is increased and the bore reduced but then the force will be smaller instead and the radius larger.

Do design small combustion chambers on an large bore, small stroke engine for a high compression ratio is a diffucult task if the flow should be high. However you design it, it will have some downside. Therefore a large stroke will be needed to reach higher compression ratios.

Diesels can't work at high engine speeds, they won't have the time needed for the combustions. Like all engines they have a torque curve that at higher engine speeds will be negative, that is, it will need power from another source to run at those speeds.

As far as I know, that WW-II Jeep engine wasn't a high compression engine and was designed to run on very poor quality gasoline. Okay, I'm not an engine design engineer, but I do have a question. Lets say that we have two 4 cylinder engines. One engine had a small bore and long stoke. The other engine has a large bore and short stroke. Both engines are designed to produce exactly the same horsepower. Which one has the greater torque and why?

As far as I know, that WW-II Jeep engine wasn't a high compression engine and was designed to run on very poor quality gasoline. Okay, I'm not an engine design engineer, but I do have a question. Lets say that we have two 4 cylinder engines. One engine had a small bore and long stoke. The other engine has a large bore and short stroke. Both engines are designed to produce exactly the same horsepower. Which one has the greater torque and why?
If you make two engines like this, the one with the shorter stroke will need to work at higher engine speeds, it will therefore produce a higher power. The torque will be pretty much the same for both engines, however, since the large bore engine run at a higher speed that engine will likely have a slightly lower efficiency and therefore produce a little less torque. However, that difference will be very small.

my understanding of stroke is that higher stroke engines with smaller bore will produce more torque at lower rpms than small stroke, big bore engines with the same displacement but they will produce around the same peak power also at lower rpm. is this correct (I'm asking you johan because i know you always provide good insight)