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why does a diesel have more torque than a petrol?


Dave
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If you take similar sized engines eg 2.0l, why does a diesel engine produce more torque than a petrol equilivent.

And what is the relationship between torque and bhp. I realise that torque increases in proportion to bhp, but how do they relate?

Cheers

Dave

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1. A gas engine compresses a fuel-air mixture and then ignites it with a spark. A diesel engine compresses just air, to such a high pressure and temperature that when fuel is then injected, it ignites automatically with no need for a spark. This is the fundamental difference between the two engines.

2. ...so, a diesel engine has to do more compression than a gas engine does, to get the fuel to ignite. Conversely, a gas engine cannot do as much compression as the diesel engine does, because the fuel-air mixture it is compressing would ignite too early, at the wrong moment. Remember the diesel engine just compresses air, so it doesn't have that problem.

3. ...so, the piston in the diesel engine has to travel further, in order to compress the air more. So the piston stroke is longer in a diesel engine.

4. ...and, a longer piston stroke means a larger diameter crankshaft. Assuming the force coming from the gas piston and the diesel piston is equal, then the diesel piston has a longer lever arm and is turning its crankshaft with greater torque (but, necessarily, at fewer revolutions per minute). The gas piston may be delivering the same power, but it is delivering it by turning a crankshaft at more revolutions per minute, with less torque.

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Horsepower = torque x rpm / 5252, so power/torque graphs should always intersect at 5252 rpm.

It's somethig to do with toque being turning force and power being how much work is being done; a more torquey engine turning at the same rpm is liberating more bhp becase it's doing more work.

Why diesels have more torque, I don't know, but I guess it's to do with the energy value of diesel against petrol - diesel being harder to ignite than petrol, but it probably containing more energy per millilitre than petrol.

Diesel engines often rev lower than petrol engine due to various requirements like injection and ignition timing. If there wa s away to make diesel as combustible as petrol, diesel engines could rev higher and liberate more power than an equivalent petrol engine.

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diesels have more torque, I don't know, but I guess it's to do with the energy value of diesel against petrol - diesel being harder to ignite than petrol, but it probably containing more energy per millilitre than petrol.

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If there wa s away to make diesel as combustible as petrol, diesel engines could rev higher and liberate more power than an equivalent petrol engine.

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OK, or perhaps that other stuff about cranks and piston travel.

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Theres a limit to the revs that a diesel can run due to the physical properties of the oil, (i.e thick), and also that it is slow to combust... hence you will never see a high revving diesel engine 169144-ok.gif

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My V8TDI is running 0-30 fully synthetic oil pengy.gifthinest oil you can put into car engines coffee.gif At UK air temps that is.

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[ QUOTE ]

If you take similar sized engines eg 2.0l, why does a diesel engine produce more torque than a petrol equilivent.

And what is the relationship between torque and bhp. I realise that torque increases in proportion to bhp, but how do they relate?

Cheers

Dave

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This may help answer your question:The answer to your question has to do with the way the two engines are designed. Your 11 liter diesel engine has a long stroke. That means that the piston is traveling a relatively long distance up and down in its cylinder on each cycle. A racing engine, on the other hand, has a short stroke. The piston in a racing engine has a large diameter for the engine size, and it goes up and down a relatively short distance on each cycle. This means that a race car engine can run much faster -- up to 15,000 RPM in a Champ Car engine -- but has relatively little torque. A large diesel engine usually cannot get above 2,000 RPM, but has huge torque because of the long stroke. The torque is what lets your engine pull a huge load up a hill.

So why does an engine with huge torque and low maximum RPM get a low horsepower rating? If you have read the article entitled How Horsepower Works, then you know that one horsepower is equal to 33,000 foot-pounds of work per minute. By this measure, one horse can raise 33 pounds 1,000 vertical feet in a minute, or 330 pounds 100 feet in a minute, or 3,300 pounds 10 feet in a minute, and so on.

What an engine naturally produces, however, is torque. Think about one piston in a gasoline engine. When the gasoline ignites, it pushes on the piston, and the piston exerts pressure on the crankshaft, causing it to turn. The crankshaft feels some number of foot-pounds of torque in the process. There are three variables that affect torque:

The size of the piston face

The amount of pressure that the ignited fuel applies to the face of the piston

The distance the piston travels on each stroke (therefore the diameter of the crankshaft). The bigger the diameter of the crankshaft, the bigger the lever arm and therefore the more torque.

There is a direct relationship between horsepower and torque. You can convert torque to horsepower with the following equation:

HP = Torque * RPM / 5,252

That 5,252 number, by the way, comes from dividing 33,000 by (2 * pi). Imagine taking 33,000 foot-pounds and walking it around in a circle rather than a straight line. For example, if you took a 10 foot pole and attached it to a vertical axle, the circumference of its circle is:

circumference = 10 * 2 * pi = 62.8 feet

If one horse is pushing on the pole with 100 pounds of force (1,000 foot-pounds of torque), it can move the pole at 5.25 RPM. Torque and horsepower are directly related to each other.

You can see from the horsepower equation that high RPM values favor horsepower. If you take an engine with a certain torque and run it at very high revs, it can generate lots of horsepower even though its torque hasn't changed at all. A racing engine can produce relatively low torque, but because it can rev so high it gets a great horsepower rating. A big diesel has huge torque, but "gets no respect" in terms of horsepower because it cannot ever get above 2,000 RPM. This "makes sense" -- if two engines produce the same torque, the one that can do it more times per minute does more work and therefore has more power.

The difference in maximum RPM ratings also tells you why trucks need so many gears. A race car engine might idle at 1,000 RPM and can accelerate to 15,000 RPM -- a multiplier of 15. A big diesel might have a multiplier of only 2 or 3. Because the RPM range between minimum and maximum is so small on a diesel, there needs to be lots of different gears to keep the engine in its productive RPM range at any speed.

169144-ok.gif

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Theres a limit to the revs that a diesel can run due to the physical properties of the oil, (i.e thick), and also that it is slow to combust... hence you will never see a high revving diesel engine 169144-ok.gif

[/ QUOTE ]

My V8TDI is running 0-30 fully synthetic oil pengy.gifthinest oil you can put into car engines coffee.gif At UK air temps that is.

[/ QUOTE ]

I think he means the 'fuel oil' tongue.gif

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[ QUOTE ]

[ QUOTE ]

[ QUOTE ]

Theres a limit to the revs that a diesel can run due to the physical properties of the oil, (i.e thick), and also that it is slow to combust... hence you will never see a high revving diesel engine 169144-ok.gif

[/ QUOTE ]

My V8TDI is running 0-30 fully synthetic oil pengy.gifthinest oil you can put into car engines coffee.gif At UK air temps that is.

I think he means the 'fuel oil' tongue.gif

[/ QUOTE ]

Diesels like piss. wink.gif

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[ QUOTE ]

Theres a limit to the revs that a diesel can run due to the physical properties of the oil, (i.e thick), and also that it is slow to combust... hence you will never see a high revving diesel engine 169144-ok.gif

[/ QUOTE ]

I did wonder if a certain amount of pre-heating and fine atomisation through multi-point direct injection (like the new Audi TDIs peizo injector system) could help up the rev limit a little bit.

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My C600 really doesnt get above 3000RPMS redlines at like 4500!! Its out putting just over 600 Lb/Ft TQ.

Its great for pulling up trees, suffocating people standing by the side of the road, and um....well yeah LOL. Transit man and Iveco drivers shrink when this is about!!

EDIT: 610 pounds-feet of torque at 1,600 RPM and 325 horsepower at 2,900 RPM from 5.9 liters of displacement

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