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Old 30th September 2009, 17:07   #16
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Originally Posted by bj96 View Post
In a diesel only air is compressed first, which increases its temperature enough to ignite fine droplets of diesel, which is forced into the combustion chamber later. Rest all are very similar to petrol engine.
Hope this was simple (and correct) enough to explain the main differences.
That was good. Can you please clarify my following doubts?

1. The principle explained implies that all diesel engines necessarily have to be four stroke, right? If it is so, how is it that even at a lower rpm the diesel engine develops more torque? Is it do with the higher thermal efficiency of diesel only, or is there a better way by which the engine converts the piston movement into torque? (BTW, I now understood why diesel adulteration is an accepted practice - the engine seems be built to use low quality fuel!)

2. If diesel (as fuel) and diesel engines are more efficient, why cannot smaller engines be built - my understanding is that the diesel engines are typically above 1 litre capacity.

3. Heavier parts = heavier engine = nose heavy (front engined) vehicles. How is the Centre of Gravity managed?
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Old 30th September 2009, 17:50   #17
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1. The principle explained implies that all diesel engines necessarily have to be four stroke, right?
Not quite true. There are 2stroke diesels too, but usually, they are used for large monsters like locomotives, ships etc. Two stroke diesel - Wikipedia, the free encyclopedia

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If it is so, how is it that even at a lower rpm the diesel engine develops more torque?
It is due to heavier flywheel/crank-shaft, it generates more moment-of-inertia translated to torque.

E.g. hold a 1kg weight in your palm with your right arm stretched and try to rotate your arms like a Kapil Dev. Now try the same act with a tennis ball. Do you feel the difference? 1Kg ball, you can't rotate as much fast as a tennis ball but you need to apply more force to rotate your hands. Faster the rotation, more is the power (movement per unit of time), but NOT more torque (force). More weight more torque. Makes sense?

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Originally Posted by vrprabhu View Post
2. If diesel (as fuel) and diesel engines are more efficient, why cannot smaller engines be built - my understanding is that the diesel engines are typically above 1 litre capacity.
Circa 80s or so- Enfield Bullets used smaller diesel engines. But it made measly power of 5 HP or so. Smaller diesel engines can't generate as much power, I think and its weight/power ratio will be sub-optimal for a good commercial vehicle application.

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3. Heavier parts = heavier engine = nose heavy (front engined) vehicles. How is the Centre of Gravity managed?
Probably by making chassis, suspension, floor, rims etc heavier, we can lower the CG.

Last edited by bj96 : 30th September 2009 at 17:54. Reason: removed bold formatting
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Old 30th September 2009, 21:29   #18
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Since, petrol engine parts are lighter, they can spin faster than diesel counterparts. Hence, a petrol engine RPMs are higher than diesels.

As diesel engine parts (flywheel/crankshaft) are heavier, they can "twist" more- more torque than petrol engines.
The high torque is not due to heavier parts but high cylinder compression/pressure and longer stroke.

The reason why diesel engines revv low is partly bcoz the parts are heavier and also bcoz the longer stroke requires the piston to travel a much greater distance to complete one cycle. Long stroke engines achieve high mean piston speeds at much lower rpms compared to short stroke engine running similar piston speeds.

Eg: An engine running 100mm stroke will reach piston speeds of 10mtrs/s@3000rpm but an 80mm storke engine only runs at 8mtrs/s@3000rpm.

Shan2nu

Last edited by Shan2nu : 30th September 2009 at 21:30.
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Old 30th September 2009, 23:19   #19
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The high torque is not due to heavier parts but high cylinder compression/pressure and longer stroke.
I guess you may be right. For simplicity (for illustration) let us assume no friction, gravity etc.

Torque(t) = force(f) x moment arm(r).
f = compression (c) x area (a)

t = c x a x r

Hence, looks like torque is proportional to compression. Is this reasoning correct?

This also means to generate so much compression you need to apply so much more torque to fire the engine. That's why diesels use more amps and a heavier startup battery to crank the motor?

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The reason why diesel engines revv low is partly bcoz the parts are heavier and also bcoz the longer stroke requires the piston to travel a much greater distance to complete one cycle...
This is talking like a seasoned bhpian.

Last edited by bj96 : 30th September 2009 at 23:30. Reason: added note on cranking
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Old 30th September 2009, 23:39   #20
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I guess you may be right. For simplicity (for illustration) let us assume no friction, gravity etc.

Torque(t) = force(f) x moment arm(r).
f = compression (c) x area (a)

t = c x a x r

Hence, looks like torque is proportional to compression. Is this reasoning correct?
Compression/Pressure is only one part of the equation. High cylinder pressure exerts more downward force onto the piston, but what really decides the amount of torque is the combination of force applied and the stroke (moment arm).

You could have 2 engines exerting the same amount of force on the piston, but you will still see a higher torque figure on the one that has a longer stroke.

What a heavy flywheel does is store loads of rotational energy which gives you a feeling that the engine is producing more torque, but if you were to take an engine with a 10kg flywheel, dyno test it and dyno test it again with a 20kg flywheel, the dyno figures wont show an increase in torque.

The simplest example of a flywheel is the sugarcane juice machine which has that huge wheel on one side. When the guy turns the wheel and lets go, the stored rotational energy keeps the wheel rotating for a period of time, allowing him to feed the sugarcane into the machine without having to constantly turn it.

Had that flywheel not been there, the machine would have stopped working the moment he stopped turning it.

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Old 1st October 2009, 14:26   #21
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Circa 80s or so- Enfield Bullets used smaller diesel engines. But it made measly power of 5 HP or so. Smaller diesel engines can't generate as much power, I think and its weight/power ratio will be sub-optimal for a good commercial vehicle application.
Thanks, bj96 - that's nicely done.

I didn't exactly have a two wheeler or car in mind when I asked the question (I distinctly remember a diesel engined motorbike which was pretty popular those days - Swaraj (used to be called "soooraj") - and used to be a sight seeing a two wheeler getting filled with diesel!) - I was thinking about the generators (Portable ones). All the bigger generators have diesel engines whereas the smaller ones run on kerosene / petrol. So, if diesel is a more thermal efficient fuel, I thought smaller diesel engined generators would be better choice.

Curious - because, there are some places where there is power cut for more than 4 hours. Now, even an UPS rated 5 hour stand-by doesn't give supply for more than 4 hours continuously. And, smaller generators can't charge these UPS's (bigger ones are not viable - spent 40,000 Rs. on diesel alone one day when it was imperative to keep the show running!). What other way can be found to charge the UPS?
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