Engines and RPM. Heres my theory

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Hello everyone, I am not very skilled with engines but heres what i think that would be pretty cool. Lets take a regular v6 2.5 liter engine it will redline at abut 6,500 rpm producing lets say 225 horespower. Now I am into a hobby of medel 1/10 scale race cars that run on nitro fuel and are controlled by a radio controller. My engine in the model is the size of your fist, it has 2.6 hp, it is a 3.5 cc engine, and to the point it has 36,000 RPM. Yes it runs on fuel like cars do except it has only one piston and runs on glow plugs instead of spark plugs. There is a 2 speed transmission in the model and it can go up to 50 mph. Do you think engines in cars should have this much RPM so you go faster or is this a waist of time. Im only 16 so the idea can be stupid. But if you think about it why not have engines rev to 30,000 rpm before switching gears.

posted by  Kris

Why not have engines rev up to 30? Because the pistons for one couldn't handle it and just the fact that the parts are moving at 30 thousand revolutions per minute, doesn't that sound like a lot to you?

posted by  99integra

just remember that on a smaller engine, the pistons don't need to travel anywhere near as far as the revs get higher.

F1 engines rev to about 20 odd thousand.

posted by  windsonian

Yeah but 30, that is just impossible

posted by  99integra

HAHA ^^ What he said.

posted by  hondaman

Its good to imagine though :wink2:

posted by  99integra

never say impossible.... if you'd told someone 200 years ago that we'd be making cars that drove 100km/h for extended periods without animal power, what do you reckon they'd have said?

posted by  windsonian

They would say what the hell is 100km/h in MPH :laughing:

posted by  99integra

hmmm... so how do they get the model engines to rev over 30,000 RPM? But how about if the regular car engines reved up to lets say 12,000 rpm? What im trying to say i guess is that maybey companies should try to invest in making their engines faster instead of bigger.

posted by  Kris

That would mean the torque would take a while to kick in, correct?

posted by  99integra

depends on the torque profile of the engine.

btw: Kris, keep refining the idea and asking questions .... that's the way people come up with groundbreaking ideas :thumbs: People will tell you why stuff won't work, and then keep chipping away until it does.

posted by  windsonian

well if it would max out at 12,000 rpm Wouldent it have to idle at around 1,500-2,000 RPM to stay on? because since the max is higher then everything wil have to move slightly faster to keep up. So when your idling at 1,500 and you put it into drive it will even out a little. But i think this is more like a turbo where it gives more topend then lowend, would probobly work best for liek nascar since their moving at a very fast speed and high rpm's. So something with torque would need really large pistons so when the fuel gets burned in the combustion chamber is that right? well i mean on top of the piston it gets egnited by the plug. So if you have a lot of fuel and a large piston its gonna make a big boom making more torque than hp right?

posted by  Kris

The thing about it is that at high rpm you don't need much torque... and if the car is revving that high, you're gonna get to a decent torque region (at the wheels) very quickly as the engine revs up.

One thing to remember is though ... if you take a standard engine and just rev it higher, you'll use more fuel.

posted by  windsonian

Also to make this kind of RPM the piston size would have to be smaller in order for it to go up and down this fast. If you use the right material and desighn the piston and crankshaft right i think it can be done. If an engine the size of your fist can spin at 36,000 RPM why cant a larger version of it do it? Also that brings up a point about the idle speed. If you put smaller pistons they will be lighter so it will idle at a higher RPM. So once you put the clutch down you are going to be moving at a good speed just idling leaving the car in drive without any gas or with a manual transmission in first without any gas so you might have bad acceleration in first gear since the rpm will need to go up so much until you can reach redline and switch, but then once its time to go into second gear you are amost at 12,000 rpm and you switch it will give you a real big kick in the a$$ and so on when you switch with the other gears. I am just wondering how powerful a engine like this could be, since well your lawn mower has 3 horespower and look at the size of its engine and my engine the size of my fist has almost 3 hp and it has one small and a lawn mower has a mugh bigger piston.

posted by  Kris

Remember you were saying about bigger engines having more low end torque. That means more force on each piston stroke.... more stress on engine components.
Combine this with higher revs and you get more cycles of this stress occurring each minute .... higher chance of fatigue failure / lower fatigue life.

Unless every component is designed below the fatigue threshold (not all alloys even have one), doubling the revs will halve the life.

posted by  windsonian

And they are both singles correct? The lawnmower can't rev to 36000 and stay togeather like your tiny engine can because of rotating mass and inertia. HP = Torque x RPM. The lawnmower makes much more torque than your fist motor but it can't rev as high because it is bigger. the crank rotatating measures your rpm correct? Think about that distance. Take your arms and hold them out and spin yourself and try to stop. Then try to spin yourself with your arms held in. You have less inertia. Just like the crank. The longer the stroke the harder each rpm is on your engine. 10k rpm on a sportbike is not the same as 10k in a Corvette. The engine is much smaller therefore it can rev higher.

posted by  VG30DE

This shows how hp and number of cylinders are related just as torque is to displacement with a certain number of cylinders.

posted by  VG30DE

And even if normal sized engines (~.5L/cyl) engines COULD rev that high, they can't breathe. You would need MASSIVE valves and lots of 'em. Has anyone gotten past 5 yet or would that negate the advantage of having more valves by making them too small?

posted by  VG30DE

That gave me a good idea... If the RPM is higher than it will cause stress and the engine will not run too long, so there has to be something to let it move this fast but not take all the stress out on the crankshaft and rest of the engine. How about compression, a LARGE compression ratio to slow the piston down when it is going down so the crankshaft wont get totally broken in half. Let me explain it a little better, When the piston comes down from the egnited fuel pushing it down there is little or no air in the combustion chamber. so it goes slower and slower because it has to force itself away from the top and to the bottom but since there is no air it goes slower and slower. But then at that point the cam shaft opens a hole for the air to get in. Insted of a camshaft, at the bottom of the pistons walls a hole for air and fuel to get in to egnite again. on a diffrent oppasite side a hole for the exaust to get out of. here is a diagram of this. If yu dont understand I will try to explain it more.

http://www.rc-airplane-world.com/images/engine-cutout.jpg

RC car nitro engines are two-cycle (or two-stroke), whereas most larger engines are almost always four-cycle - with the exception of things like small lawnmowers or scooter engines.
Being 2-cycle means that it really is a very straightforward thing, as there are no valves, cams or lifters that are found in a 4-cycle engine.

Two-cycle means that the piston just needs to complete one down-stroke and one up-stroke within the combustion chamber to complete the process of drawing the fuel and air in and then converting the ignited mixture into raw power and also expelling the exhaust gases.

The piston will keep on moving up and down (hopefully!) so long as the fuel/air mixture is being ignited, but how does the whole process start? Well, the fuel/air mixture needs to be introduced into the crankcase at the bottom of the combustion chamber before anything can happen.
This is done by 'priming' the engine, which usually means holding a finger over the carb venturi or exhaust opening while slowly turning over the engine by hand, or just squirting fuel directly into the carb from a plastic bottle with a fine nozzle.
Either action makes sure that fuel is put into the crankcase.

Now that fuel is in the crankcase and has mixed with air, the glow plug needs to heat up and the piston needs to move up and down. So to start, the nitro engine needs to be turned over manually, which is either done with the pull cord, if it is a pullstart engine, or by an electrical engine starter.
At the same time, the plug is made to glow red hot by connecting a special battery to it for a few seconds; the core of the plug heats up immediately.

With the fuel/air mix sitting in the crankcase, it needs to be moved up to the top of the chamber and ignited. It's moved to the top as the piston moves down; as it does so it pressurises the area of the crankcase where the fuel/air mix is and forces it up small ports, or channels, that run up to the top of the chamber. The mixture is then ignited by the glow plug on the top of the engine.
Unless the engine is in poor condition or badly tuned, it should fire up after just a few seconds of being turned over.

When the piston is at the point just before the mixture ignites, it is covering the exhaust ports which run from the sides of the combustion chamber through to the silencer. When the piston has almost reached the top, the fuel/air mix ignites because of the glow plug, and the resulting explosion forces the piston back down again. On the way down, the exhaust ports are uncovered by the piston and the burnt gas mixture leaves the chamber via the ports.
The momentum of the spinning crankshaft makes the piston move upwards again and the whole process starts over.

Although the glow plug was made to glow red hot initially by a battery, once the engine has begun to run then the high temperature inside the combustion chamber keeps the plug glowing. If the plug fails, then the fuel/air mix will not get ignited and the engine will stop.

posted by  Kris

the issue with lowering the stress on the crank is that the force exerted by the pistons is the torque created by the motor, decreasing it decreases the efficiency of the engine. You're saying use the gas pressure to absorb the force ... thus less torque.

posted by  windsonian

Don't forgtet, it's not only piston mass, but piston SPEED. YOur engine turns at 25k plus, but the piston SPEED is scaled down (how far does it travel in one revolution? i.e. what's the stroke x 2?)

Remember, a, say, Ford 5.0 liter has a 3 inch stroke. that means teh piston moves 6 inches vertically per revolution, averaging 1500 ft per minute at 3000 rpm. 30,000 rpm would equate to 15,000 feet per minute, or 170 mph average (it's higher in the center of that stroke, and drops to zero at the top and bottom). Your R/C engine has a stroke (approx .5 inch) that equates into a piston speed of only 250 feet per minute at 3000 rpm, and 2500 ft per minute at 30,000 rpm or 28.3 mph average!!!! Let that sink in. The piston speed in the 5.0 liter engine passes that nitro engine piston speed at a mere 5000 rpm!

posted by  ChrisV

Damn Chris you never cease to amaze me :wink2:

posted by  99integra

momentum, momentum!!!

posted by  windsonian

look up COATES spherical rotary valves, new head design theyve been experimenting with, i think they got a 351W ford motor up to 14,000rpm b4 they bottom end exploded

posted by  BanffAutoSpa_ap

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