bibo

Ok so generaly car engines turn much higher RPM than airplane engines.

Does anybody know why ?

I would really like to hear some opinions.

HPIDUDE

you know that there is a engire forum right?

mziadeh

My Feedback: (1)

Basicly car motors have a shorter stroke which enables them to turn the higher rpms.
For more details go to the engine forum.

cardriverx

well airplane engines are 4 stroke, which limits them to how much RPM they can turn. The complex design and lots of moving parts means that if they where reved too high, the rockers or springs or valves etc. would just snap. I'm not sure about the stroke, but if the airplane engines do have a longer stroke, that is another reason why.

say bye

cardriverx, all i can say is huh.
first off there are 2 and 4 stroke engines for airplanes.
second there are many applications where a 4 stroke will actually out rev a 2 stroke

i'm fairly new to rc (about 3 yrs) but i've set-up race engines 2 and 4 for many years. rpm ranges have many variables on a 2 stroke engine. stroke length, port timing, expansion camber size and length, flywheel wieght and size and rotating mass in general, compression ratios, squish area, and if you have an ignition then ignition timing plays a role.

46u

My Feedback: (3)

They all so make 4 stroke car and truck engines.

MrBurpsalot

I think all U guys are all wet, the real reason a plane engine turns much slower is because it is swinging a propeller.
Try bolting a prop on any "glow" engine and they all will slow down because of the air drag.

HPIDUDE

or mabye they're tuned differently and they produce more tourque

bibo

so you're saying an airplane engine will rev like a car engine if put in a car ?

MrBurpsalot

They would certainly turn faster than in a plane, maybe not as fast 40,000rpm that we see in some "High end" engines ,
But yes they would run much harder and faster swinging less mass(i.e. an input shaft verses a fan blade).
Over heating would be the plane engines biggest enemy.

MrBurpsalot

I really don't get it..... In the R/c truck and car world we refer to to .25 and larger engine as a "bigBlock" and this engine displacement size in the R/c plane world is rather small.

MrBurpsalot

Yea man, i agree with you too!

bibo

Yes because the manufacturers claimed low RPM for airplanes engines is already measured without the propeller so definetely it's not just the drag of the propeller.

bibo

Say bye , out of all the variables you mentioned which variable is mainly responsible in making an engine high revving ?

odysseus

A lot of it has to do with gearing. Almost all airplanes have the prop attached directly to the crankshaft. This means there's a 1:1 ratio between prop rpm and crank rpm...if the engine is spinning at 15,000 rpm, than so is the prop. Car/truck engines have gears to slow up the rpms from the engine crankshaft to wheels. The benifit is torque. We use the rpms in combination with the gearing to create good acceleration(actually move the vehicle) and maintain a controllable top speed. If there was a 1:1 ratio in our cars, than acceleration would be slowwwww, but top speed(if you had enough space to get it there) would be rediculously fast(thats why top speed records are set at the Boniville Salt Flats...they need the space). The weight of the car offers more load on the engine, thus requiring the gear reductions in the transmission and primary drive. The load produced by the prop in the airplane is less...there is still a load, but it's effects on our expectations on how the airplane performs is different. This all means that the engine in an airplane demands the use of power found in a certain rpm range to provide controllable, smooth flight. Cars demand performance in acceleration and top speed that require gearing to achieve. Essentially, we can let the rpms of the car fall to idle and the worse that can happen is the car rolls to a stop. Airplanes require a certain rpm to just get off the ground and if they drop below that, than the airplane is now a glider and coming down at some point. On the flip side, incredibly high rpms in the airplane can become useless. There is a point where no matter how fast the rpms are, no reasonable gain in performance is achieved. So, they don't bother reving an engine so high when all they have to gain is a potenial mechanical failure in prop or engine integrity. You can only spin a prop(or rotor for the helicopters) so fast before it shatters!

gasoline hpi

Also, an airplane engine operates at its maximum rpm for a lot of the time, while in a car you get on and off the gas all the time, an airplane is kept at wot a lot of the time. because of this, as well as the reasons odysseus said, airplane engines are designed to operate at lower rpm, they have different intake and exhaust port, and induction timing. while it is possible to derive more power per a given displacement by turning more rpm, an engine can make decent power at lower rpm if it is designed to operate at lower rpms. like odysseus says, there is no reason to take risks by running an engine at high rpms to make maximum possible power when that is not necessary. in a car, .2 more horsepower can give you that 5 mph you need to beat the competitors car, but with a plane consistent performance is more important than having the best possible performance. that and, like odysseus says, if you turn a prop too fast it breaks.

say bye

in an rc nitro engine i would say that port timing and expansion chamber shape and size are the biggest determining factor in the rpm range of a givin engine.. i've never flown a plane or worked with there engines but i can tell you that the prop which you can buy in different pitches will smooth out and load an engine heavily compared to a car. that is rotating mass and wind resistance. if you installed a prop on a car engine it would smooth out and lower the rpm's .it would also make it less prone to stalling and have a smooth rpm transition from idle to top rpm's.

mziadeh

My Feedback: (1)

You guys are missing the whole point, the single most contributing factor to the max rpm of any engine is the stroke of the piston. The longer the stroke, the lower the maximum engine rpm but it will produce more torque. The shorter the stroke the higher maximum engine rpm, it will produce more horsepower but less torque and that's where the gearing comes into play on the trucks/cars. This is not only true in RC applications but for engines and general. Diesel engines have a long stroke in comparison to a simalar displacement gasoline engine, have a red line of about half that of a gasoline engine, produce over twice the troque and only about half the horsepower.

END

say bye

stroke is a factor but it is not the main factor on a 2 stroke engine. as i see from your post you are using 4 stroke engines in your post and in that engine format it is more important. however in a 2 stroke format it is not the biggest factor. in designing a 2 stroke engine you figure in what rpms you want the engine to be most efficient, ie: most power and tuners will call this " on the pipe" , "powerband" or" the sweet spot". unlike bigger 2 strokes that can use valves to change the size of the exhaust port or even in watercraft that use water injection systems to change the perceived size of the exhaust to give a broader powerband. the rc engine only has port timing and a set exhaust expansion camber. this gives it a narrow range that it is at maximum potential. with a rc car engine you want a engine that will scream and give good over rev characteristics. with a plane you have a massive flywheel weight ierop that as rpms grow so does it's resistance . you will only have over rev on an airplane while in a dive. with this being the case you want the engine of an airplane to have good torque and a lower rpm range because the faster you spin the prop the less efficient it becomes so you get the engine to that rpm range by port timing expansion camber size stroke and even carb size.
i don't think the point was missed
one more thing, i used the word powerband and in my years i have had people ask how to adjust it. you can but this is not a part it is merely a description of where any engine is running the best, most efficient that it can.