High vs Low RPM motors

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Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

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Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>
My understanding (possibly lacking and/or wrong) is that for one, the combustion pushes the piston for a longer amount of time. But, what I think is more significant is that by having a longer stroke, the journals are further from the center of the crankshaft, giving the piston/connecting rod more leverage. Because torque is rotational force, by having more leverage on the crankshaft it is easier to rotate, so there is more torque from a longer stroke.
Remember, these are the musings of a high school student, so some or all of it may be dead wrong.

Danny,

Just because a long stroke engine has a larger crank radius doesn't mean it's going to produce more torque. The torque it produces is a result of the combustion process, which can have vastly different levels of effectiveness (efficiency) in different engines. Also, a longer stroke means a longer time spent on the compression stroke where you are losing piston energy.

Or, for a given capacity;

A long stroke engine has a smaller bore, = smaller piston area, = smaller force acting on its longer lever arm.

A short stroke engine has larger bore, = larger piston area, = larger force acting on its shorter lever arm.

Bottom line is that for a given capacity the torque is (approximately) the same regardless of bore-stroke ratio (ie. ~100Nm/litre, (+/-20% from pulse tuning)).

But, as pointed out above by Owen (and in more detail on another thread recently) all engines have a workable maximum mean piston speed of about 20-25 m/s, so a short stroke engine can rev higher, and so produce more power.

Z

(PS. F1 engines for the last ~20 years have B:S ratios >2 (B = ~95mm, S= ~45mm), yet produce similar torque to any other engine of the same capacity, and have similar "racing engine" maximum mean piston speed of 25 m/s. The decades long movement to shorter strokes and higher revs is a direct consequence of seeking more power from a given, rules-mandated, maximum engine capacity. It is not the most efficient way of making power (frictional losses), but in motorsport expensive + inefficient = good !)

Just adding to what has been said and demonstrate this with some simple equations:
Torque = Force X Distance
Torque = Pressure X Area X Distance
Torque = Pressure X Area X Stroke/2 (Crank pin length will be half stroke length due to crank slider mechanism)
Note here that Area X Stroke = Volume
Torque = 1/2 * Pressure * Volume

So unless you increase MEP, you cannot increase torque for a given volume. MEP will be affected by several things such as volumetric effiency, ignition timing, compression ratio, fuel type, A/F ratio etc. These are all tunable parameters or affected by the design of intake/exhaust (cams/ports/valves too).

IMHO there is more hype than deserved regarding high rpm vs. low rpm engines. People often speak of a high-rpm engine having absolutely no power at lower rpms, while if you actually overlay a couple dyno charts it will only be down by maybe 20%. While 20% is a significant drop, it certainly isn't "absolutely no power." However, when driving the high rpm engine it may seem that way, as it drives just as lame as the low rpm variant until it "hits the cam" and things get wild.

A great example of this is comparing the Buell XB-12 with the Buell 1125R. They were both made at the same time, and had similar displacements (1200 vs. 1125cc). The XB-12 had essentially a hopped-up Harley 45º air-cooled v-twin that they managed to get to survive reliably to over 6,000 rpm. The 1125R had a modern Rotax DOHC water-cooled, fully balanced 72º v-twin that happily spun to almost 11,000 rpm. Both were tuned to Erik Buell's specifications, so you really can't get a better comparison of a high-rpm vs. low-rpm motor. Here are the two dyno curves:

http://i167.photobucket.com/albums/u...vsXB12dyno.gif

Another misconception is that high rpm requires power over a narrow range of rpms. There is a lot that goes into cam design that determines the breadth of the power curve; a high rpm motor does not necessarily have to be peaky. You will get more output from a peaky engine, but it won't be as usable. Regardless, that is a function of cam design and is largely independent of rpm (although high-rpm engines gain more by being peaky...). What I find even more interesting about comparing these two is that the high-rpm motor actually flattens out more on top than the low-rpm motor. It actually has a broader powerband!

My personal preference is a highly linear output like the two engines shown above. Power comes on very gradually and predictably as a function of rpm. You can short-shift it and hold a higher gear in sweepers or on slaloms for smoother torque management, and just lean into it on exit, and if you're really good you can mesh the progression into the power band with corner exit. It just makes it easy to be smooth IMO.

Something you also have to take into consideration is that the engine is just a part of the whole car. So the weight, the center of gravity and the dimensions are also very important.
As already said by the others, an engine that revs higher, will need a shorter stroke. With a shorter stroke you will have a smaller crankshaft. So it can be positioned lower, which also lowers all the other internals. Additionally you can make your connecting rod shorter, so your cylinder head moves down, too.
And although your cylinder head will have to be a bit wider, I would say that this engine would also be lighter.

In F1 (until 2005), they did put a lot of effort into the engine development. Not just because of the power, but also because of the cog, the weight and the dimensions.

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Originally posted by Paul Gerisch:
With a shorter stroke you will have a smaller crankshaft.

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Smaller in diameter, but wider (in case of a multi-cylinder engine), as you mentioned below.

Quote:

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Originally posted by Paul Gerisch:
Additionally you can make your connecting rod shorter, so your cylinder head moves down, too.

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Keep in mind that conrod length affects the engine's secondary balance. So, with shorter stroke --> higher rev range, your conrod should not be TOO short...

P.S.: Welcome to this forum, Paule http://fsae.com/groupee_common/emoticons/icon_wink.gif

I love adambombs graph. This shows the sacrifice to a higher rpm engine quite well. Because friction goes up with RPM, you shouldn't expect to get the same torque at higher rpm, but ultimately torque is most closely linked to displacement. Stroke and Bore have an impact on where the peak torque rpm is, but very little influence on the peak amplitude.

There are certainly engines out there that do not live up to their displacement potential. For instance many stationary, and high duty cycle engines. These are engines the designers worked to increase reliability and lifespan on over outright output and shouldn't be used for comparison.

But if you look at racing engines from AMA superbike through drag racing pro-stock engines and everywhere in between, the torque vs displacement points make a pretty straight line.

It would be rather interesting to take those two engines as tested, fit restrictors to them, retune them, and test them again.

It could well be, that the old Harley banger with it's higher Ve at lower rpm out torques and maybe even out powers the Rotax.

The Rotax probably has much larger ports and valves, and much more radical valve timing. Strangled right down with a restrictor, it is never going to be able to reach anything like 11,000 rpm.

And at the lower max airflows limited by the restrictor, it could quite likely be operating at a severe disadvantage.

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Originally posted by Slim:
A high rpm engine will of course be smoother, but I think in the range of engines you find in FSAE that you wouldn't really get to the point where the driver feels a choppy power delivery. However, singles and (low speed engines?) can be harder to start and idle because there is a much greater speed and power variation throughout one cycle. Which I've heard can become significant to combustion dynamics at low speeds.

It might be because at low rpm you lose a much larger percentage of combustion heat to your engine block. Your heat rate is roughly constant and so the longer a stroke takes the more heat will be lost. If your engine goes fast enough, this affect will be reduced but you will begin to lose more energy to friction loses. Peak thermal efficiency is somewhere between these two extremes.

Longer stroke motors also see higher piston accelerations and higher forces. Which means they need to be build heavier and are likely to accelerate more slowly than a lighter high rpm engine. Although, I don't know what happens when you apply a suitable gear reduction and the fact that your longer stroke engine is operating at lower speeds/accelerations.

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High RPM engines doesn't have anything to do with how smooth the operation of the engine actually is. The "smoothness" of an engine is determined by the number of cylinders, firing order, balancing factor of the crankshaft and any additional balancers added to the engine (be it a balance shaft or balancing gears). These items create torsional vibration and torsional bending of the engine block as the corresponding cylinders are fired. This is where firing order plays a role as the arrangement of "bangs" setup torsional waves along the crankshaft and into the block.

What do you mean by choppy power delivery? Do you mean how the power delivery feels between a single cylinder and a 4 cylinder? Or do you mean from a tuning aspect.

Heat transfer between the combustion process and the cylinder block have nothing to do with RPM (per say), but to do with ignition timing (that is why I said per say, spark timing is dependent on RPM and your mixture. So I guess you are right. However -&gthttp://fsae.com/groupee_common/emoticons/icon_wink.gif. Generally at lower speeds your engine has a low spark advance value as there is more time available for the peak pressure to reach an optimal 15-16 degrees ATDC. As you increase your operating speed of the engine more advance of spark timing is required to allow for peak in cylinder pressure to remain close to the previous value of 15-16 degrees ATDC. Now since you've added more advance onto your spark timing you expose the cylinder walls to the flame front for a longer period of time increasing the heat transfer between the flame front and the cylinder walls. Therefore you are loosing energy from the combustion process to raising the heat of the coolant/cylinder walls/block. Is it possible to retard the timing at higher RPM's to generate a faster combustion rate? I'll let you think about it.

Jan_Dressler is correct with what he mentioned. Just one thing, this probably isn't the case in FSAE as a restrictor is already found upstream of the ports, but if your ports are to small you have the potential of reaching coke at the throat of the port. Port design is also extremely important aspect of engine design, however more often so the majority of the engine geometry is already predetermined from the manufacture leaving little room for additional improvements in gaining fluid energy through the ports. Also try to eliminate swirl in port injected engines, the centrifugal forces of the air-fuel mixture swirling into the combustion chamber will wet the cylinder walls removing oil from the wall = trouble http://fsae.com/groupee_common/emoticons/icon_wink.gif.

I am also surprised that no one mentioned anything about the momentum of the air/intake pressure waves and its correlation to RPM? Or maybe I missed it.