Engine RPM on small engines and big engines

Discussion in 'Technical' started by Pando, Dec 11, 2004.

  1. 1) Why is it that a normal passenger car 2.0 litre inline-4 engine has a redline at e.g. 7500 RPM, a GSX-R bike 1.0 litre inline-4 has a redline at 13 000 RPM, and a 3.5 ccm RC-car engine has a redline at 40 000 RPM?

    2) Is it a rule of thumb that, the smaller the engine, the higher the redline of the engine will normally be?
    If so, why is it like that? Is it because a bigger engine has heavier moving parts and thus cannot be moved back and forth as quickly as lighter parts (too much mechanical stress/wear - they break? What happens?)? But bigger and heavier moving parts - doesn't bigger also make them stronger? Maybe not equally stronger to keep the redline at the same level as the weight goes up?

    3) How does F1 engines go to 18 000 RPM (and race bike engines go to what? Over 20 000?)? I've heard before, "short stroke, light and good parts, only has to last one race". I think the first two are ok arguments, but the last one is a little strange, how can it work perfectly fine for the entire race, then break? How does it break? Does the connecting rod between the piston and crankshaft break or something?
     
  2. A smaller engine needs to rev higher to make the same amount of power, that or forced induction.

    F1 engines rev so high because they have very short strokes and big bores, and they can breathe really well. I think I remember they've got over 100% volumetric efficiency.
     
  3. It is a combination of the three reasons you said, depending on the application. A short stroke is the main reason. The shorter the stroke of an engine is, the lower the maximum piston speed will be at a given rpm. This in turn means that the piston and connecting rod will undergo smaller accelerations at that given rpm. Since it is generally the forces caused by these accelerations that cause failure, you would be able to run the engine with a shorter stroke at a higher rpm, until the resulting forces would be equal.

    As for only having to last one race that would depend on the kind of race the car is built for. If you buy a production car, it is engineered with very high factors of safety, with the idea in mind that it should last tens or hundreds of thousands of miles. In most forms of racing, engines are used for perhaps several hundred miles, and are then rebuilt, so they can have lower factors of safety without having problems.

    And as for reference, I don’t think any one liter racing bikes go up to 20k, the RC211v goes up to 16,000 and I would guess that most equivalent bikes are somewhere around that.
     
  4. It's easier to rev an engine higher if it has a shorter stroke.
     
  5. many things contribute to an engine revving high(er) and the most major one is stroke. a longer stroke will increase piston speeds and G loads on the piston(and the rod) while a shorter stroke does the opposite.

    however, bore has an effect on this aswell, though not nearly as significant. the larger the bore(for the same stroke) the heavier the piston becomes, increasing reciprocating mass and again limiting the revv-ability of the engine(but not nearly to the same degree as increasing stroke).
     
  6. About the one race thing in F1 (soon to become two races). The pistons/engine don't necessarily work fine for the race and then break, but after being pushed so hard for the duration of the race, they're worn down by all the stress of moving at such high speeds. If a team wanted to, they could just rebuild an engine and run it again for the next race, but that would mean all the parts weren't in perfect condition, and so the engine would have a lower power output.
     
  7. There's also significantly lower relative reciprocating mass in smaller engines, meaning that con rod loads etc. are all lower relative to larger engines of the same bore/stroke.
     
  8. Bigger engine = longer stroke, which results in harder acceleration and thus more stress.
    But also, bigger engine = stronger parts doesn't it? Does a twice as big engine have twice as strong parts?
     
  9. Why are not normal performance engines (the ones used in normal sports cars and street legal race bikes) made with very short stroke - as in F1? Not made to last only a fraction of what a normal engine does, but made with a much shorter stroke to allow for more RPM?
     
  10. No, doubling the amount of material doesn't affect strength if you're also doubling the volume.

    EDIT: I think.
     
  11. MrMortigi, i meant race bikes not street legal, do those really have a lower redline than the F1 3.0 litre engines?
    If so, then why? Why don't they make the stroke very short and thus be able to reach extreme RPMs because their engines are already very small relative to F1 engines?
     
  12. the strength of a part is MOSTLY related to how the part is engineered. a well designed part made of a inherently weaker material(relatively, not extremely weaker) is definitely capable of surpassing the performance of a weakly engineered part of a more appropriate material.

    making a part twice the strength sometimes doesnt need more material and can even be done with less material, it lies in the design(aside from material as mentioned).

    as for the short stroke thing, the shorter the stroke the lower the torque output. this makes the engine not well suited for street use. extremely high revving engines are also more prone to failure and wear and tear because they are high strung.

    remember that a stronger heavier part is not what you always want. a well designed race engine for example will have just enough material in the pistons, conrods, crank and block to sustain its regular intended use. the components are made with as little mass and material as possible within the guidelines of the governing bodies(if any restrictions are in place). if you make a piston very robust but make it weigh an extra 20% it can actually end up being weaker when in use, because the mass of this piston is now being reciprocated at hundreds of cycles per second making it experience rediculous acceleration/g loads where the extra mass is now exerting far greater strain on the piston itself aswell as other connected components(conrod, crank, wrist pin, various bushings and bearings etc).
     
  13. The RC211v, which I mentioned above, is Honda's race bike used in MotoGP.
    I do not know exactly why the engines in Formula One can rev higher although speaking purely on the topic of stroke, I know that the Honda CBR600 F4 (a 600cc road bike engine) use a stoke of 42.5mm, while according to the Ferrari Formula 1 book (which has tons of detailed info on their 2000 car), the stroke of the engine used in 2000 is 41.4mm, slightly shorter than that of the bike engine.
    I know that this does not prove anything, but in order to increase the volume of the engine from 600cc to 1000cc the total dimensions would have to increase, and logic would suggest that the stroke would likely increase at some proportion compared to the bore. (The F4 is a four cylinder engine while the RC is a five cylinder, but each cylinder would still have to be larger.)
    Additionally, although there are benefits to short strokes, there are also downsides. As the stroke decreases for a specific engine volume, the bore must increase which can lead to combustion problems. At very high engine speeds, the combustion flame does not have time to propagate throughout the cylinder and you do not get complete combustion. This was a problem that Ducati had with their MotoGP enignes, which is why they moved from V-Twin engines, to V-4s.
    Additionally with motorcycles, one deals with packaging restraints because the engine must be mounted below the driver and in general you have less room to work with than with a Formula One car.
    These are merely my thoughts on the topic, and if anyone has more to add please do so.
     
  14. Most likely not. There's obviously not a general rule to describe every situation, but if anything I'd say that a smaller engine likely has greater relative strength than a larger engine. For one thing, as you start using more and more material, you introduce the chance for more and more imperfections (e.g. tiny cracks), which can all contribute to reduced material strength.
     
  15. A bigger engine doesn't nessisarily mean a longer stroke. Example: the Chevy 302 has more displacement than the 283, but both use the same stroke length crankshaft. The difference is the cylinder bore, the 302 uses the 327/350 block's 4.000" bore, and the 283 uses the early SBC block style with the 3.875" bore.
     
  16. also

    momentum decrease from a mass decrease will allow those higher rpms.

    F1 pistons are very minimized in weight and construction and very very light.
     

Share This Page