http://forums.24hoursoflemons.com/viewtopic.php?id=4861&p=1 This guy is making some truly radi(c)al changes to a MR2. Worth looking at from an engineering perspective. Image Unavailable, Please Login
Living proof that you can't argue with a sick mind.... A/C engines turn at lower RPM's so that the prop won't have too high a tip speed. Not much power for a given amount of displacement at 2500 rpm's. But displacement equals weight so the power to weight ratio pretty much stinks. They will learn this hard lesson the first time they try to spin it more than 3,000 rpm and they throw a rod out the side of the block. And then you are going to have cooling issues unless you put a lot of horsepower into a fan.... Sorry, but I don't see a pony at the bottom of that pile. If you really wanted to do something different, you can't beat a small block Chevy for power to weight ratio but I guess some people want to do something totally off the wall, for whatever reason.
Well, there's a custom-built transfer box in there that will spin UP the RPM into the automotive trans -- they don't plan on spinning the radial too high. Check the thread, there's some cool engineering going on in there.
Quick rule of thumb for any normally aspriated 4 cycle engine: You typically get about 1 ft pound of torque per cubic inch of displacement. If you go to higher speeds that can fall off, but the one for one rule is usually a good place to start. Since HP is tq X rpm/5252, using a big motor at lower RPM doesn't offer much other than more installed weight when compared to a higher speed machine for the same horsepower. Power to weight ratio is all about displacement and RPM. Airplanes are a unique design problem in that the output shaft speed is artificially low because that's the speed the prop needs to turn to avoid the tips of the prop from going supersonic. If you turn the engine at the speed of the prop, you elimnate the heavy, expensive and unreliable gearbox. There are lots of examples of geared engines in aircraft, but the weight of the gearbox, combined with the higher stress in the engine that results from turning it faster, often leads to reliability problems. Bigger, slower turning engines have been used in aviation since the Glenn Curtis got rid of the bicycle chains that the Wright Brothers used to gear down their engines. That doesn't mean that they make much sense in a car. When you put an aviation engine into a car, the engine makes big torque at low RPM's but it doesn't make big horsepower, and the gearbox gets bigger and heavier since the torque of the engine is greater. In this case they are using a speed increasing gearbox so that they can use the automotive gearbox and drive the axle, so that's just more weight they are adding to the system. Like I said, there isn't a pony in that pile other than doing something that is different to be different. A higher speed smaller displacement engine is a far better way to go.
Hmmm.... really? While you equation is correct it also points out the that HP is nothing more than a calculation and "torque" is the actual ability to do work. When I hear HP numbers quoted what I think of is how much fuel is going to be burned, which more often than not requires more fuel be carried because range is a sales "feature". Greater torque output does require parts that are up to the task, but so does greater RPM... Newton's second law and all, for the extreme example take a look at the cost of a modern Formula One engine. Generally greater RPM means accelerated wear. Further, there is more to power output than just displacement and RPM...forced induction, VE, oversquare vs undersquare etc all play roles and require an engine system designed to take advantage from intake to exhaust...then to cover a wider "speed" range a transmission is needed as in most ground based applications. The old drag racing axiom that speed is a question of money is just as true in the air as on the ground. The bottom line is there is no free lunch.
Torque isn't work, or even the ability to do work, it's simply a force...Think of it this way... Torque is the force, but torque times speed (or really force times a distance) is work. You can make big torque, but if you don't have rotational speed it doesn't mean much. Acceleration comes from the ability to do work, not from torque, but from torque times speed. Forced induction is a totally different case, with FI there are no rules, and that's why FI was so hard to control in F1. Engines could make as much power as you were willing to pump air through them, despite the fact that the thermal efficiency was falling like a brick. But, if you look at peak torque, that's going to occur where the VE is maximized (and be about 1 ft lb/cubic inch). That's where the rule of thumb comes in, and that's why I said, if you spin the engine at really high speed the torque will drop off, because at higher speeds the VE will fall off due to pumping losses. And undersquare/oversquare doesn't seem to make a big difference, as to the peak torque, still it's about 1 hp/cubic inch. Oversquare tends to move the torque peak up to higher speeds, and it moves the piston speed down so there is less wear and friction, and that allows the higher speed engine to make more power. For every course there's a horse. For piston airplanes I like big displacement slow turning engines with no gearboxes. That has provided the best reliability and proven performance for the last 60 years. It's interesting that there have been literally thousands of engines that have been developed or are flying in homebuilts that have tried other routes, the Lycasaurus type engine is still is the best thing we have out there to turn props. For cars, smaller higher speed engines are lighter and have proven to be more efficient and provide better performance. Since you are going to have a gearbox anyway, a smaller high speed engine works better in cars.
I respect to right of people to do as they wish with their own cars......yet I cant' imagine a worse idea than what i'm seeing in that 1st picture. Good Luck. Geffen.
Your definition of torque is correct, I was over simplifying...however to be literal it is the measure of a force's tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force and the force vector, and quite literally in Latin it means to twist. I agree with your statement about FI, and disagree on the undersquare/oversquare. As a side note, the old 1to1 adage really is no longer applicable...in my current assignment 2-1 is a baseline from 5 years ago. Lastly, I understand you comment about the "Lycasaurus"; but I don't think it is the best or even close. To me it is the best that the development dollars alloted at the time have come up with, and really in the last 20 years (or more) how much real development money was allocated to engines? Gas was cheap and there was no way we'd run out...oops. Currently aviation engine suppliers act like MFI, DBW, VVT are alien technology and require vast amounts of unobtanium! The reality is these antiques are going the way of the dinosaurs before. A real boost in development and testing needs to happen soon, but with the failure of the last round of diesels (hello Thielert) I fear we are in for more of the same. How can you economically justify the cost of R&D for a shrinking market? I don't see any upswing in piston GA until some core problems are resolved. Sorry for the thread jack, I now return you to one ugly MR2.
If it doesn't work in the MR2, then he can always put the radial engine in a motorcycle. Image Unavailable, Please Login
Since F1 engines don't make more than 1.5 ft pounds of torque per cubic inch of displacement and the 458 (the current highest specific torque/displacement in production) makes about 1.5, I'm curious as to what 4 cycle normally aspriated engines that were around 5 years ago that could make 2 ft pounds of torque per cubic inch. That would have a MEP of almost 290 lbf/sq in.... Turbo, yea, sure, but NA? I just don't have any experience with anything like that. If you were describing horsepower per unit of displacenment yea, that's very doable, but not torque...
I've flown a Ryan pt-22 with the Kinner and it's a load of fun. They sound like a Harley with fouled plugs. We had to prop start the plane and it had impulse magnetos.
