[anupamjain2k]

Originally Posted by vina

[/i]I don't know where the limit on rpm comes from, I guess the limiting factors vary from engine to engine, but your argument is confusing to say the least. For a given displacement and bore size, stroke length is fixed, irrespective of rpm, and CR can still be increased by, for example, turbocharging (in fact CR is equal to ratio of volume at TDC and volume at BDC, merely by reducing the volume at TDC you can increase the CR, without changing displacement and, by the arguments you have given above, the rpm).

For a given displacement, CR and fuel burnt, short stroke (vs. long stroke) also means that the force on the piston is higher - almost exactly cancelling the effect of a shorter stroke - torque will be the same.
I'm not sure about your gear ratio stuff (though in theory it is correct) - there are several applications where a low rpm engine is connected via gears to generate higher rpm outputs (diesels mainly) but do you know of any significant commercial application where the other way round also happens (e.g. a 8000rpm engine used with a reduction gear arrangement).

I know they do it with turbojets in military helicopters, but don't know any other application.

vina, For a given displacement, and bore size, stroke length is fixed (irresp. of rpm) and CR is also fixed. You can increase it (subject to engine block tolerating the pressure build-up due to higher CR, and the engine not misfiring etc.) As you've rightly mentioned, CR as ratio of volumes, the bore being fixed, its merely the distance traveled by piston between DCs. Reducing the volume of TDC would thus make the fuel-vol ignited lower and hence change the overall engine. Having said that, these are broad rule-of-thumbs, and like everything else in engineering, they are essentially trade-offs. You can always gain an element at the cost of another.

For a given disp. (bore X Stroke), CR is fixed. If you change CRs, stroke length changes and the engines aren't comparable. However, considering hypothetically that to not be the case, you have answered your query. Since the force on the piston is higher, short stroke would produce more power, though, with smaller piston travel (Resulting from short stroke), torque would be lower.

Essentially, the gearboxes used in vehicles are all reduction gearboxes (Try seeing the ratio of wheel rpm, Vs engine rpm). And this is gen. achieved in 2 steps - Gearbox, and Differential (essentially, constant ratio reduction gear). So, when saying low rpm engines trying to generate higher rpm outputs, its a relative comparison and essentially means, trying to be in the engine's power-band using the best possible gear-ratio.

[DirtyDan]

Originally Posted by vina

Torque and power are of course related but they are different...

And that is a contradiction and not a contradiction? At any rate, this is mostly off topic.

[vina]

Originally Posted by DirtyDan

And that is a contradiction and not a contradiction? At any rate, this is mostly off topic.

power = torque*rpm - torque and power are related but different. where is the contradiction?

[Karthik1100]

i have a small confusion here. wider bore helps to gain more torque or longer stroke helps to gain more torque? as i have observed specifications of some cars/bikes having wider bore produces more torque and cars/bikes having longer strokes are the fastest. isn't wider bore consumes more fuel to push the piston than smaller bores? isn't longer strokes helps in gaining more RPM to push the car/bike faster? sorry if my way of thinking is wrong

[vina]

Originally Posted by anupamjain2k

vina, For a given displacement, and bore size, stroke length is fixed (irresp. of rpm) and CR is also fixed. You can increase it (subject to engine block tolerating the pressure build-up due to higher CR, and the engine not misfiring etc.) As you've rightly mentioned, CR as ratio of volumes, the bore being fixed, its merely the distance traveled by piston between DCs. Reducing the volume of TDC would thus make the fuel-vol ignited lower and hence change the overall engine. Having said that, these are broad rule-of-thumbs, and like everything else in engineering, they are essentially trade-offs. You can always gain an element at the cost of another.

Vol@BDC = displacement + Vol@TDC

CR = Vol@BDC/Vol@TDC = 1+ displacement/Vol@TDC

For a given displacement you can easily increase the CR merely by reducing the Vol@TDC, so for a given displacement CR is not fixed.


Anyway, this has nothing to do with Sutripta's original question - how are CR and rpm related?

Originally Posted by anupamjain2k

For a given disp. (bore X Stroke), CR is fixed. If you change CRs, stroke length changes and the engines aren't comparable. However, considering hypothetically that to not be the case, you have answered your query. Since the force on the piston is higher, short stroke would produce more power, though, with smaller piston travel (Resulting from short stroke), torque would be lower.

As I wrote above (and in my previous post), even for a given displacement changing the CR is merely a matter of changing the cyl volume at TDC. So I don't see where the "For a given disp. (bore X Stroke), CR is fixed" coming from.

Also at any given rpm if one engine produces more power it will also generate more torque, power=torque*rpm (use appropriate units) so once you fix the rpm it is not possible to increase one without increasing the other.

Can you explain how would a short stroke engine will generate more power but less torque at the same time?

Originally Posted by anupamjain2k

Essentially, the gearboxes used in vehicles are all reduction gearboxes (Try seeing the ratio of wheel rpm, Vs engine rpm). And this is gen. achieved in 2 steps - Gearbox, and Differential (essentially, constant ratio reduction gear). So, when saying low rpm engines trying to generate higher rpm outputs, its a relative comparison and essentially means, trying to be in the engine's power-band using the best possible gear-ratio.

I don't think anyone will disagree. However you claimed "So, short stroke engine on low rpms would have very little torque, and hence such engines would typically have to employ high gear-ratios to compensate." in an earlier post.Now my question is do you know of any vehicle with a low-stroke engine that employs this kind of gear ratios, and can you mention a comparison between these and some other vehicles to clarify? While what you claim should be certainly possible, as I mentioned earlier, except in systems using turbines I haven't seen very heavy reduction gearing to gain back low-end-torque

[ex670c]

Hi Guys,

I think, and based on a few observation.

"Between two engines of similar displacement, the petrol version generates more Torque."

Let look at a few engines.

Petrol F4-134 --2187/2193cc - 16Kgm approx.
Diesel XDP4.9 -- 2112cc - 12Kgm approx.
Petrol K20A -- 1998cc - 21kgm approx.
Petrol K20A -- 2157 - 23Kgm approx.
Diesel 2.0SDI -- 1968cc - 14Kgm approx.
Petrol P-40 6cyl - 3956cc - 26-28Kgm approx
Diesel W04D 4cyl - 4009cc - 22-26Kgm. approx


Regards,


Arka

[vina]

Originally Posted by ex670c

Hi Guys,

I think, and based on a few observation.

"Between two engines of similar displacement, the petrol version generates more Torque."

Let look at a few engines.

Petrol F4-134 --2187/2193cc - 16Kgm approx.
Diesel XDP4.9 -- 2112cc - 12Kgm approx.
Petrol K20A -- 1998cc - 21kgm approx.
Petrol K20A -- 2157 - 23Kgm approx.
Diesel 2.0SDI -- 1968cc - 14Kgm approx.
Petrol P-40 6cyl - 3956cc - 26-28Kgm approx
Diesel W04D 4cyl - 4009cc - 22-26Kgm. approx


Regards,


Arka

very interesting - are these diesels turbo-diesels? If not then there is nothing to wonder.

[ex670c]

Originally Posted by vina

very interesting - are these diesels turbo-diesels? If not then there is nothing to wonder.

Hi Vina,

Let me correct it as between two naturally aspirated engines of similar displacement, the petrol version generates more torque.

Some more

Diesel Peugeot TUD5 - 1587cc - 10.7Kgm approx
Petrol Suzuki G15A - 1493cc - 12.8Kgm approx
Diesel VW 1.6NA - 1600cc 10Kgm approx
Petrol Suzuki G16B - 1590cc 13Kgm approx

Chrysler LA - Petrol Magnum 5.9L V8 -- 44-47Kgm Approx
Cummins 6B Diesel - 5.9L I6 -- 32Kgm approx

Usually a Turbo-Charged engine is consider to have an effective displacement = Stock X1.5

So 2.0L TCIC has an effective displacement of 2.0X 1.5 = 3L

But the real advantage diesel have is with CRDI & TCIC, is that they can replace much larger engines.

Regards,

Arka

[vina]

Originally Posted by ex670c

Hi Vina,

Let me correct it as between two naturally aspirated engines of similar displacement, the petrol version generates more torque.

Some more

Diesel Peugeot TUD5 - 1587cc - 10.7Kgm approx
Petrol Suzuki G15A - 1493cc - 12.8Kgm approx
Diesel VW 1.6NA - 1600cc 10Kgm approx
Petrol Suzuki G16B - 1590cc 13Kgm approx

Chrysler LA - Petrol Magnum 5.9L V8 -- 44-47Kgm Approx
Cummins 6B Diesel - 5.9L I6 -- 32Kgm approx

Usually a Turbo-Charged engine is consider to have an effective displacement = Stock X1.5

So 2.0L TCIC has an effective displacement of 2.0X 1.5 = 3L

But the real advantage diesel have is with CRDI & TCIC, is that they can replace much larger engines.

Regards,

Arka

So the diesels you mentioned are in fact all NA, as I suspected.

regarding "Usually a Turbo-Charged engine is consider to have an effective displacement = Stock X1.5" - wouldn't this depend on the initial boost the turbo/intercooler can give to the air pressure (i.e. overall O2 in the cylinder)? I mean for a higher pressure turbo wouldn't the 1.5x be larger?

What limits the max obtainable pressure?

[ex670c]

Originally Posted by vina

So the diesels you mentioned are in fact all NA, as I suspected.

regarding "Usually a Turbo-Charged engine is consider to have an effective displacement = Stock X1.5" - wouldn't this depend on the initial boost the turbo/intercooler can give to the air pressure (i.e. overall O2 in the cylinder)? I mean for a higher pressure turbo wouldn't the 1.5x be larger?

What limits the max obtainable pressure?

Hi Vina,

I don't know the finer details, but 1.5 or 50% improvement is one of the Formula's accepted by the FIA.

As an engine undergoes forced induction, the torque increases and the Max Rpm reduces (500-1000rpm).

Regards,

Arka

[Born2Slow]

Let me confuse every body little more. What if i said that the power developed by an engine (theoretically ofcourse) has nothing to do with its cubic capacity but the torque produced is directly proportional to its cubic capacity. Power is directly proportional to the breathing ability of the engine (disregarding efficiency loss etc)
Let me explain - power is energy produced / unit time.
So disregarding things like mechanical efficiency,combustion efficiency and for that matter lets forget the engine altogether and just assume we are burning the fuel in open air. So to produce certain power we always need X amount of air and y amount of fuel burning per unit time.
So its up to us whether we burn x/100 and y/100 (cubic capacity) fuel and air 100 times a second (rpm) or x/1000 and x/1000 fuel and air 1000 times a second. So torque as we know it is a way to quantify the power produced per unit cycle of the engine and rpm the repetition of that unit cycle. So theoretically any capacity of engine can produce any power as long as you rev them hard and the torque produced is directly proportional to its cubic capacity.

Lets take an example of the bullet 500 it has a bore of 84 mm and stroke of 90mm. It produces about 22 bhp @5400 rpm and about 3.5kgm @3000 rpm. what happens if you increase the stroke of the engine to say 180 mm and keep the rest of the engine same. So by doubling the engine capacity it has now become 1000 cc but will it produce more power the answer is no, will it produce more torque -yes but at a lower rpm. why? Because it still has the same breathing characteristics (valves/head etc) and in a unit time say 1 sec it can only breathe the same amount of air/fuel mixture. But the torque goes up as now per unit cycle it is burning more air/fuel mixture and rpm goes down as it takes more time to fill double the capacity using the same breathing. so you will get an engine now which produces same power at lower rpm and double the torque at half the rpm. (please note in practice the characteristics wont exactly change as I mentioned because the way engine breaths but will be close).

Well diesel or petrol or otto cycle or thermodynamic cycle the basic theory remains more less the same. Now we will have to consider the efficiency of the process and specific heat capacity of the fuel also. Also just remember high CI does not increase torque by itself but increases the efficiency of heat conversion to work thereby increasing torque ,so if we increase the CI from 10 to 20 the torque wont double but will increase a few percentage of that. Petrol and diesel has same stoichiometric ratio so same amount of air (by weight) can burn same amount of petrol or diesel by weight. I saw in some earlier post that diesel has more energy than petrol but that is wrong (a liter of diesel has more energy than a liter of petrol due to higher density but by weight petrol has more energy per gram).

Lets consider two engines each 1 ltr , one petrol other diesel (lets assume similar max Volumetric efficiency). Lets assume 30% efficiency for petrol and 40% efficiency for diesel due to higher CR of diesel. so only considering this if petrol produces 3 torque diesel will produce 4 torque. But petrol has more energy per gram than diesel so torque is compensated to some extent in petrol. Most of all here I have assumed stoichiometric ratio but diesels are able to run much leaner so reducing torque and increasing efficiency. So we get something like say 2.5 torque for NA diesel and 3 torque for na petrol. Due to the small bore and long stroke design of the diesel it is not able to maintain volumetric efficiency as the rpm goes up so will produce almost half the power of the similar NA petrol engine (or 3/4 maybe due to the state of tune). Now what happens when we turbo charge the diesel, the torque doubles (and becomes much more than petrol) and the power also doubles and becomes similar to a NA petrol. Now people start comparing the two as they have same power and say diesel has more torque than petrol. It is like comparing a 1.5 liter efficient petrol engine with a low power 3 liter engine of same power and saying that 1.5 liter produces less torque than 3 liter engine. What you should ask is why is the 3 liter engine not producing more power.
So when you are comparing a NA petrol and turbo charged diesel of similar size the question you should ask is "why the hell is the diesel not producing more power after being turbo charged so that we can compare the torque to a more powerful petrol engine" instead of asking "why diesel produces more torque than petrol".

[bzr77k]

Why is it not surprising that a thread on "Torque vs BHP" talks more about the difference between "Petrol and Diesel"

Anyway, i have a question on the Torque/BHP curves... why does the torque of an engine fall from a peak?
I understand that the torque would be low at very low RPMs as the amount of energy generated is low due to lower RPM. But then, as the RPM increases, what are the factors that cause the torque curve (and as an effect the power curve) to fall?

i can think of mechanical restrictions due to valve operations as one possible cause... the time taken by valves to open/close once the command is issued is fixed (a few microseconds (us)). This fixed time would be less important at lower RPMs as the overall valve operation time would quite large (a few ms against a few us) and at higher RPMs this time becomes more significant and imposes a restriction on the overall operation time of the valves...

is my understanding above correct? if no, then where am i going wrong?

also, any other factors that cause the torque curve to drop from the peak?

[Sutripta]

Hi,
Does a diesel use all its air?

@Arka, I think the torque figures for the F134 are grossly exaggerated.

I think Heywood has a list of BMEPs for various types of engines. Spike?

Regards
Sutripta

[ex670c]

Originally Posted by Sutripta

Hi,
Does a diesel use all its air?

@Arka, I think the torque figures for the F134 are grossly exaggerated.

I think Heywood has a list of BMEPs for various types of engines. Spike?

Regards
Sutripta

Hi Sutripta,

The Torque Figure for F4-134 --
http://www.film.queensu.ca/cj3b/Phot...4DynoGraph.gif

Please explain IMEP & BMEP.

Regards,

Arka

[sgiitk]

While diesels do have a higher torque, due to higher gearing the gain on the road may not be as high as it may appear at first sight.