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Diesels low revving. Why?

Discussion in 'Motors Archive' started by Bug One, 21 Dec 2002.

  1. Bug One

    Capodecina

    Joined: 18 Oct 2002

    Posts: 10,365

    Location: Sandwich, Kent

    Ok, there's beeen some threads recently going on about how diesels have loads of torque, but cant match the power of a petrol car because they rev lower.

    Well, I think I've got a fairly decent understanding of engines. I thought that a diesel engine was very similar to a petrol, however the fuel has to be warmed before it will ignite.

    Why is it that diesels cannot rev as highly as a petrol engine?
     
  2. Conanius

    Capodecina

    Joined: 18 Oct 2002

    Posts: 12,352

    I too wonder this.. as my brothers rc car uses glow plugs.. about 30k rpm that peaks at..
     
  3. Event Horizon

    Wise Guy

    Joined: 18 Oct 2002

    Posts: 2,169

    Location: Let me out!

    Simple, you cant advance the ignition on a diesel ;)
     
  4. Liverpool-Lad

    Wise Guy

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    Location: Liverpool - UK.

  5. CGrieves

    Wise Guy

    Joined: 25 Oct 2002

    Posts: 1,861

    Location: Clapham Junction

    Much higher compression is the main reason- diesels don't have a spark plug as such... the compression of the mixture causes it to detonate on it's own. In a petrol car this would be disastrous (pinking) and result in melting pistons, but in a diesel it's by design. The resultant explosion is at a lower temperature if I recall, but due to the forces and compression involved the piston, conrods etc have to be beefier, thus much heavier internals in the motor=lower revs.

    Also I believe diesel vaporises differently to petrol, so the mixture is more difficult to get into the motor in high volumes.

    Turbos normally bring a petrol motor closer to detonation, which is why turbo motors usually run a lower compression ratio. But in a diesel we want detonation, so turbos suit diesels in particular (as does nitrous oxide for the same reason believe it or not)

    EDIT: Come to think of it, another reason is that diesels inherently produce their peak power typically much lower in the rev range (i.e. more torque... remember torque is simply power output per revolution) so there's no need for them to rev so high anyway as power tails off dramatically at higher revs, particularly in bigger diesels.
     
    Last edited: 21 Dec 2002
  6. Bug One

    Capodecina

    Joined: 18 Oct 2002

    Posts: 10,365

    Location: Sandwich, Kent

    So in essence.

    - Internal components are heavier due to higher compression.

    - Detonation works on compression not ignition so isn't as spontaneous.

    - Efficiency.


    Sound right?
     
  7. energy

    Wise Guy

    Joined: 22 Oct 2002

    Posts: 1,293

    Location: Chingford

    Also, as a diesel requires high compression to ignite the fuel, the piston must have a long stroke so that it can sufficiently compress the fuel. This is what gives a diesel its good torque output (perhaps there are other factors involved as well), and also what limits its maximum rpm.

    For example, a very small engine as found in R/C cars and the like can manage 30k rpm as the piston only has one or two centimetres to oscillate through.
     
  8. Simon

    Capodecina

    Joined: 21 Oct 2002

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    Location: Berks / Moscow

    SOrry to correct you but power doesnt exist.

    Its simple a value to qutify the rate torque is applies... or work done.

    1bhp is (PooP mind blank :confused: its late) IIRC 33,000 lbft/min

    The diesel produces ace torque figures but as it only up to about 4.5 k the rate it can applie work is much lower than a revving petrol. hence the lower power figure.

    Imagine a tight nut on a car wheel, if you had all the torque in the world, great?..... no not if no revolutions of the wheel brace is occuring, work is not being done only a force applied

    The Deisel engine is like that strong, but cant work fast enough to create good power.

    Honda Vtec on the other hand has medicre torque but applies it so fast with the revolutions of the fast spinning engine that work done is very high and hence the high power output.

    Diesels have a compression ratio, circa 20:1 this requires strong compoinents and a robust block, hence the heavy weight, the engine cant rotate fast with all that mass and it doesnt particularly help that you cant optimise ignition points ;)

    Hope this is slightly conherant, im drunk and its late :cool:

    Jonny
     
  9. Bug One

    Capodecina

    Joined: 18 Oct 2002

    Posts: 10,365

    Location: Sandwich, Kent

    The last paragraph was what I'm looking for. I understand why its not making as much BHP, ie lower revs. Just wondered why it could only do low revs.

    The answer seem to be because the engine is more rhobust/heavy and less control over the ignition.

    On that line of thought, what if expensive materials were used to construct the engine. IE Alloys (prefferably not magnesium ;) ), titanium, etc, and it was made very light, but equally strong.

    ARAIK RC cars can rev higher not only because of weight/size but because they use 2 stroke engines. 2 stokes have a lot simpler combustion sequence, which allows for very high RPM. I would of thought that is similar for a rotary engine? :confused:
     
  10. ALD

    Gangster

    Joined: 18 Oct 2002

    Posts: 372

    Location: Devon

    Im sure its got something to do with the rate of combustion.
     
  11. Dogbreath

    Capodecina

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    Location: Devon

    Correct,

    Combustion, not detonation. Detonation is a bad thing to be avoided.

    Efficiency really has nothing to do with it. The main two points are as above. You can build a Diesel engine to rev high, but power at high revs is limited by the speed of combustion.
     
  12. Dogbreath

    Capodecina

    Joined: 18 Oct 2002

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    Location: Devon

    Of course you can. Ignition on a diesel is control by the injector timing, which can be advanced and retarded in the same way the spark in a petrol engine can be.
     
  13. Dogbreath

    Capodecina

    Joined: 18 Oct 2002

    Posts: 16,651

    Location: Devon


    No, the stroke only has a very indirect bearing on the comrpession ratio. CR is defined as swept volume/unswept volume. Swept volume = bore * stroke.

    You can have an extremely short stroke engine that has a far higher compression ratio than a long stroke engine. In fact a short stroke with a large bore is the prefered arangement for making power because larger valves can be fitted into the combustion chamber, and the maximum aceleration the piston undergoes is less, so the engine can rev higher without breaking.

    The diesels excellent torque is purely down to the extrememly high compression ratio they use, typicaly 20:1 or more. A higer CR gives higher peak cylinder pressures, and hence torque (and more noise).

    The thermal efficiency of an engine is also directly related to CR, so this is why the diesel has such good fuel economy. Another point in it's favour is that (most) diesels have no throttle, the engine speed/power is goverened by simply injecting more or less fuel. This means the CR stays relatively constant over the entire RPM range, and means the thermal efficiency is good even at part "throttle" unlike a car whose thermal efficiency is highest only when the throttle is wide open.
     
  14. Event Horizon

    Wise Guy

    Joined: 18 Oct 2002

    Posts: 2,169

    Location: Let me out!

    Indeed it can, ...if the compressed gas has reached a temperature that enables combustion, so you are still limited.
     
  15. I think the main reasons for the diesel engines lower revving status have been covered here now.

    speed of combution

    and

    the increased mass of the engine components.

    diesel is combusted in three stages,you have a delay period where by atomised fuel is injected in to a compressed aircharge*

    uncontrolled combustion, where the fuel mix is starting to ignite and the resulting flame front traveling through the cylinder resulting in a rapid pressure rise.

    controlled combustion, where the fuel burns correctly through out the cylinder creating a massive pressure peak.

    the delay and incontrolled points in the process are the areas that cause most concern. and modern technology and the introduction of electronically controlled common rail systems are now narrowing the gap in diesel performance by actually managing the fuel delivery in stages (pilot injection ) which brings smoother power delivery and reduced noise and emissions.

    the heavier components don't just stop with the pistons and rods. The crank assembly also needs to be strengthened. but the flywheel is also a lot heavier to help the engine run with as little lumpy'ness as possible.