[thamban]

The full write-up is attached as a Word Document.



Oversquare engine Torque is theoretically equal to Undersquare engine Torque but practically Oversquare engines are designed so that it has a stroke above minimum So for mechanical reliability. We practically see that most high revving engines typically have low low-end torque.


Now for the same RPM, N the Oversquare cylinder piston has to move less distance up and down than the Undersquare cylinder piston (So < Su). This improves the high revving performance of Oversquare engines since it easier for them to do the same RPM compared to an Undersquare engine of same displacement.

So a high-revving engine is desired to have Oversquare engines. Performance car engines and bikes need high speed capabilities that translate to high RPM because of which they are made Oversquare.

The characteristics of these Petrol engines are that it has almost flat torque curve and so a linearly increasing Power Delivery Curve with RPM. So the driving method is to drive at higher RPMs to get the most out of the machine.

Now coming to Diesel engines, the important characteristics of a NA Diesel engine is its high Torque at lower RPM. This makes it desired for applications having high inertial load, for e.g trucks, heavy machinery etc. These are not high speed application but applications with higher initial load.
This also means that NA Diesel engine has a non linear power delivery which means the engine will deliver less power at higher RPMs than at lower RPMs

Consider a Trailer climbing a hill. It needs lots of Torque at lower speeds. Now if we were to fit a petrol engine to the Trailer it has to be always kept near to the red line and also at lower gears so that we get the maximum power out of the engine. This leads to higher fuel consumption and engine stress. If the Trailer is having a NA diesel engine then it can climb at lower rpm at a higher gear, which translates to higher fuel efficiency and less engine stress.

Now if we were to fit a Diesel engine into a performance car, then we need a linear power delivery. This is where the turbo-charged engines come into picture. When we use Turbo charged Diesel engines we are actually increasing the Power delivery at higher RPMs. But inherently Diesel engines cant high revv as much as petrol engines because of uncontrolled ignition which is somewhat eliminated by direct injection, but still it lags behind Petrol engines in high revving capability.

So an Oversquare Diesel engine is to be used when there is a need of high speed operation. The Petrol engines outclass Diesel engines in high speed operational performance any day. So there is not much incentive to produce Oversquare Diesel engines except for maybe high speed boats or tugs or where you need high speed performance and huge initial load bearing capability.

[thamban]

Originally Posted by Sutripta

What is that? And why? This was the original question, and a definitive answer is still awaited. I doubt the " somewhat eliminated by direct injection" bit because pre CRDI/TDI days, IDIs revved better than DIs. So it has to be the injection method (CR/ PD) rather than the DI. Am not upto speed on CRDIs.
Sutripta

Normally Petrol engines are spark ignition which implies the timing of ignition can be controlled by the spark plug timing. For efficiency of engine the ignition has to be at the peak of compression. But diesel engines are compression ignition, which means they are not ignited by spark plugs but by the pressure and temperature formed by compression. Older carburetor engines used air fuel mixture to be compressed by the piston and it ignited on its own. This is the uncontrolled ignition. Now when its direct injection diesel is injected into the compressed air chamber (whether CR or PD). In this we can actual prevent premature ignition by changing the timing of injection which was not possible in carburetor engines. Still Diesel engine ignition is uncontrolled in the sense precise timing is never possible unlike the spark ignited Petrol engine. This needs more time to be allowed for ignition in diesel engines and there by limiting the speed or rpm.

Hope this explains.

[Sutripta]

Hope this explains.

^^^^^^

Actually, it most certainly does not Mainly because the question was "is there a problem with the combustion process in a big bore/ oversquare diesel."

The only noninjected Diesels I know of are model aero engines.

Anyway, this thread is on the relationship between power characteristics and stroke length. To keep things simple, I think its best to concentrate on NA petrols. Diesels, and the question were meant for the other (Oversquare diesels) thread.

Regards
Sutripta

[Sutripta]

I know its extremely bad manners to bump up ones own thread, but hoping for some discussion.

Let me get the ball rolling.

As I said, I have a problem with one part of conventional wisdom regarding the power delivery characteristics of short and long stroke motors. Lets get the part with which I have no problem out of the way.

Let us start with two engines, one long stroke, the other short, but otherwise similar, and with very bad breathing. And with very mild cams. ie max rpm is breathing limited, not mechanical stress related. These engines will have similar characteristics (This statement is open to challenge). We now go about modifying both for more power.

As everyone has said, at some stage the short stroke will start producing more power because it can rev higher (for the same mechanical stresses), and breathe better at those higher rpms. Rpm for peak torque will be higher for the short stroke. The peak torque figure itself will be lower. All this gives us what we have always learned:- short strokes are gutless screamers. And I have no problems with this. So does anyone else have problems with this/ have comments to make?

Regards
Sutripta

[Shan2nu]

As everyone has said, at some stage the short stroke will start producing more power because it can rev higher (for the same mechanical stresses), and breathe better at those higher rpms. Rpm for peak torque will be higher for the short stroke. The peak torque figure itself will be lower. All this gives us what we have always learned:- short strokes are gutless screamers. And I have no problems with this. So does anyone else have problems with this/ have comments to make?

If both engines are tuned to their max capabilties, then this should be true.

But bcoz different engines are designed and tuned differently, you are bound to see contradicting examples.

Ford Ikon 1.6 vs OHC VTEC is one example i've already mentioned.

Ikon 1.6
82 x 75.5
Max power @ 5500
Max torque @ 2500

OHC VTEC
75 x 84.5
Max power @ 6800
Max torque @ 4700

Shan2nu

[Sutripta]

^^^^
Not really good examples as valving (breathing) is radically different.

However, the Rocam was one of the engines I had in mind as an example for the second part of my question.

Regards
Sutripta

[Shan2nu]

Not really good examples as valving (breathing) is radically different.

Thats why i mentioned it. Having a shorter stroke doesn't always indicate a high revving engine.

But on 2 ideally tuned/designed engines, a shorter stroke should help produce more power/torque at higher revvs. The biggest prob is finding such engines for comparison.

Shan2nu

[Sutripta]

Doing the rude thing once again:- bumping my own thread. But this time I'll put my final thought down and close the thread. Promise!

I think we all agree that the long stroke ultimately cannot match the power of the short stroke. But what about the reverse. Can the short stroke match the low speed torque of the long stroke? Conventional wisdom says no, but I do not see why not.

A short stroke running the same topend (not just same cams but also valves, runner volumes and dimensions, induction and exhaust systems) as the long stroke should breathe essentially the same, and will exhibit similar characteristics. And I don't see any fundamental barriers to that. (We are not considering things like larger quench areas and HC emmisions etc as fundamental barriers).

This also begs the question:- why is the world not awash with short stroke engines (infact undersquare rules), each tuned for its respective application. Would make for great product rationalisation.

Any insights welcome.

Regards
Sutripta

[vina]

Here's a link to Heywood's book:

Download Internal Combustion Engine Fundamentals John Heywood.pdf from filesonic.com - Filestube.com - download everything

Here's an excellent thread:

A question about long-stroke or undersquare engines - Page 2


displacement matters, stroke (by itself) doesn't.

Originally Posted by Sutripta

Doing the rude thing once again:- bumping my own thread. But this time I'll put my final thought down and close the thread. Promise!

I think we all agree that the long stroke ultimately cannot match the power of the short stroke. But what about the reverse. Can the short stroke match the low speed torque of the long stroke? Conventional wisdom says no, but I do not see why not.

A short stroke running the same topend (not just same cams but also valves, runner volumes and dimensions, induction and exhaust systems) as the long stroke should breathe essentially the same, and will exhibit similar characteristics. And I don't see any fundamental barriers to that. (We are not considering things like larger quench areas and HC emmisions etc as fundamental barriers).

This also begs the question:- why is the world not awash with short stroke engines (infact undersquare rules), each tuned for its respective application. Would make for great product rationalisation.

Any insights welcome.

Regards
Sutripta

If you go through the thread I just posted, an engine maker confirms that stroke/bore wouldn't matter (all other things being equal).

Racing engine analogy is not entirely appropriate - the tradeoffs are different.

A racing engine generates its peak power at a much higher rpm than the engines you find in a regular car - the clincher is that the regular car enginer can not do that high rpm to begin with. For similar displacement and compression ratio at the same rpm a race car engine actually generates LESS power than a regular car engine ( Power=torque*rpm, so at same rpm there is no difference between higher torque or higher power)


Now to have the possibility of higher rpm as well as faster response of the engine you need lower rotational inertia (lighter flywheels anybody) - that requires you to have smaller stroke in the engine.

[Shan2nu]

I wonder what would happen if you took 2 identical engines. Same CC, CR, ,Cam, Bore, Stroke etc with the only diff being that the components of one engine has stock parts (crank, conrods, pistons, flywheel, etc) but the other one has been extensively lightened.

I feel both engines, even though idectical by design, would have very different characteristic.

Shan2nu

[Sutripta]

Hi,
A thread resurrected from the dead! A true Easter miracle! I guess it is because of http://www.team-bhp.com/forum/indian...ml#post2324497

Originally Posted by vina

For similar displacement and compression ratio at the same rpm a race car engine actually generates LESS power than a regular car engine

Now to have the possibility of higher rpm as well as faster response of the engine you need lower rotational inertia (lighter flywheels anybody) - that requires you to have smaller stroke in the engine.

In reverse order. Smaller stroke = lower speeds/ accelerations/ forces of the reciprocating masses. = higher rpm limits for same reliability.
Higher MoI will blunt throttle response (same as having a heavier car), but should have no effect on steady state power.

Regarding the race car engine generating less power than a family saloon at a given rpm needs too many qualifiers to be used as a blanket statement.

Originally Posted by Shan2nu

I wonder what would happen if you took 2 identical engines. Same CC, CR, ,Cam, Bore, Stroke etc with the only diff being that the components of one engine has stock parts (crank, conrods, pistons, flywheel, etc) but the other one has been extensively lightened.

I feel both engines, even though idectical by design, would have very different characteristic.

Shan2nu

Throttle response will feel brisker in the lightened engine. Steady state powers should remain the same. That is a function of the amount of air flowing through the cylinder.

But then again Honda in its press kit for the cbr250r says lighter reciprocating masses (valves) help combustion. Something which I don't understand.

Regards
Sutripta

[vina]

Originally Posted by Sutripta

Hi,
A thread resurrected from the dead! A true Easter miracle! I guess it is because of http://www.team-bhp.com/forum/indian...ml#post2324497


In reverse order. Smaller stroke = lower speeds/ accelerations/ forces of the reciprocating masses. = higher rpm limits for same reliability.
Higher MoI will blunt throttle response (same as having a heavier car), but should have no effect on steady state power.

Whatever be the reason, bigger bore area = higher power (Heywood's book).

Also race car engines do not have the same reliability. By design they last a race, if they last more that is a bonus (and if it lasts 100 races the designer will try to put the trade-off more towards performance next time) that may or may not be welcome.

Higher MoI will not only blunt throttle response but also, because a good fraction of the MoI will be due the pistons, effect max-rpm and hence steady state power. Though for a given rpm (if achievable) MoI of the shaft has no influence on power.


Regarding the race car engine generating less power than a family saloon at a given rpm needs too many qualifiers to be used as a blanket statement.

May be - but what I meant was that if at a given rpm engine A produces less torque than engine B then engine A actually produces less power than engine B at the rpm. Engine B however may be more powerful at its peak-power-rpm (which may be considerably higher than A's peak power rpm).

From all the other discussions it seem that race car engines at low rpm numbers produce less torque (for a given displacement) than do regular engines - ergo my comment on race car engine generating less power than a regular engine of same effective displacement (i.e. include compression ration into it)



Throttle response will feel brisker in the lightened engine. Steady state powers should remain the same. That is a function of the amount of air flowing through the cylinder.

But then again Honda in its press kit for the cbr250r says lighter reciprocating masses (valves) help combustion. Something which I don't understand.


This may be because time available for combustion is the time after the inlet valve is firmly shut and the time before the exhaust begins to open. Faster acting valves can increase this time (help combustion) and low inertia valves will open up and close off faster.

This effect will of course be relevant at high rpm only.


Regards
Sutripta

Well, I hadn't seen this thread earlier, otherwise I would have posted earlier.

[pranavt]

Originally Posted by vina

This may be because time available for combustion is the time after the inlet valve is firmly shut and the time before the exhaust begins to open. Faster acting valves can increase this time (help combustion) and low inertia valves will open up and close off faster.

This effect will of course be relevant at high rpm only.

The action of the valves depends more on the ramp rates of the cam and the quality of the valvesprings. Lighter reciprocating masses would purely help in throttle response and make for a sharp engine, nothing else.

Too much has already been said so don't know where I can chime in. Pity, a lot could've been added and a lot of knowledge could've been gained in the process lol.

[vina]

Originally Posted by pranavt

The action of the valves depends more on the ramp rates of the cam and the quality of the valvesprings. Lighter reciprocating masses would purely help in throttle response and make for a sharp engine, nothing else.

Too much has already been said so don't know where I can chime in. Pity, a lot could've been added and a lot of knowledge could've been gained in the process lol.

you are right for cam actuated valves - I missed that. Also for cam actuated valves the valves also are a part of MoI - you are right too.

Still wouldn't lighter valves allow the cams to rotate faster for similar stress generation (meaning rpm can be higher)?

This is an old thread - so even if you are repeating something there aren't that many people looking. Please do add whatever you feel you can, at the very least I'm interested to learn from you.

[pranavt]

Originally Posted by vina

Still wouldn't lighter valves allow the cams to rotate faster for similar stress generation (meaning rpm can be higher)?

This is an old thread - so even if you are repeating something there aren't that many people looking. Please do add whatever you feel you can, at the very least I'm interested to learn from you.

The spring rates of the valvesprings affect the resistance faced by the cam in turning more than the weight of the valves. But stiffer valvesprings are a necessity if you're going for higher revlimits or higher-lift cams. Lighter valves would allow you to get away with springs which are a little less stiffer, ultimately making the rotation of the cam easier, which I believe you are pointing at in your question.

Just by considering stroke between 2 different engines, you cannot conclude that one will make more power than the other, or where it will make more power reliably. The bore diameter matters, as does the rod-stroke ratio which determines dwell, side-loading force on the piston (which would impact combustion characteristics), etc.

Just to discuss the theory about shorter stroke = higher RPM reliably, the F20C motor by Honda which was made for the Honda S2000 had a long stroke (when compared to other high-revving motors) because of which set the record for the highest mean-piston-speed when revving at 9000 rpm, also being one of the highest-revving production engines in the world. This doesn't really disregard theoretical proofs as much as it shows that you can make freak motors that go against conventional logic and still be reliable.