Hi, the Mazda RX-8 is the reason I’m asking this. I’ve done some research, and I think that I know how the rotary engine works on a basic level, but I’m not familiar with the more detailed aspects. I’m hoping some of you folks can help me out.
Now, the way I understand it, rotary engines don’t have pistons (obviously), they have a triangle that rotates. Now, from what I’ve gathered, there is just one triangle that moves around and produces the power for the car. Is that correct, or are there two or more triangles that rotate on a cam of some type?
Also, the redline RPM on the new Mazda is about 9K. On my current 4cyl turbo, it’s 6-7K I think, and when I had a V-8 it was closer to 5-6K. So, why is it that more pistons causes fewer RPMs? Is it just because the more pistons, the more chance there is of the engine flying apart?
And another question, the rotary engine produces 250hp, and a similiar V-6 car gets about 250hp. How is it possible for such a small engine to produce as much HP as a much larger engine?
And finally, the RX-8 is a 1.3 liter, and the Infiniti G35 is a 3.5 liter. But the 3.5 liter gets better gas mileage? What does the liters mean in this case.
Thanks for the replys, hopefully the question wasn’t too long to read!
Just to point out a few things (I’m not an engine expert by any means) the Red line is determined but a few things, including piston travel length. My I4 1.8L toyota engine has a redline of 8300 rpm due to it’s shorter stroke…motor bike engines normally go up to 15000-18000 rpm because the stroke is very short.
Now, from what I’ve gathered, there is just one triangle that moves around and produces the power for the car. Is that correct, or are there two or more triangles that rotate on a cam of some type?
Typically there are two or three triangles, called rotors. They rotate with the crankshaft.
Also, the redline RPM on the new Mazda is about 9K. On my current 4cyl turbo, it’s 6-7K I think, and when I had a V-8 it was closer to 5-6K. So, why is it that more pistons causes fewer RPMs? Is it just because the more pistons, the more chance there is of the engine flying apart?
Sort of. It’s a matter of reciprocating mass. A lot more stuff (pistons, etc.) has to move a lot farther, and go back and forth rapidly, on a piston engine.
And another question, the rotary engine produces 250hp, and a similiar V-6 car gets about 250hp. How is it possible for such a small engine to produce as much HP as a much larger engine?
Each rotor has three sides that are involved in compression and power delivery. In that sense, each rotor is like three cylinders. For general power comparison, most two-rotor engines are roughly equivalent to four-cylinder engines. I think you’ll find the RX-8 is turbocharged, which adds to its horsepower.
And finally, the RX-8 is a 1.3 liter, and the Infiniti G35 is a 3.5 liter. But the 3.5 liter gets better gas mileage? What does the liters mean in this case.
The gas mileage has more to do with the power produced and used than with the specific engine size. The liters refer to total engine displacement, which is the volume of air that can be drawn into the cylinders or rotor chambers.
OK, some answers for you. I believe that the RX8’s Renesis engine is actually a twin rotor design, so there are two rotors rolling around in there. This ties into your last question. For many years, (I believe since the second gen RX7s), the 1.3L designation has been a misnomer. Because most of the more moden Mazda engines were dual rotor, the displacement was actually 2.6L. So, you would be comparing a 2.6L engine to the G35s 3.5L.
As far as getting better gas mileage, rotaries run notoriously rich. This is for a couple of reasons, but basically boils down to the fact that a ping, or knock, caused by a lean condition on a rotary engine, will blow the engine apex seals, requiring an engine rebuild. This is actually not such a big deal on the RX7 engines, since a rebuild entails replacing apex seals. A believe an RX7 rebuild can cost as little as $300. This rcih condition can also resul in cool flames shooting from the tailpipe of older RX7s. However, it also contributed to the demise of the earlier rotary, which had a lot of trouble passing stricted emissions regs.
More pistons causes fewer RPMs for a couple of reasons. One is that car designers don’t really care about absolute numbers, they care about performance. If the car does what they want it to, they don’t care bout the numbers. Larger engines, with usually more cylinders and larger displacement, normally have more torque, so they don’t need to rev as high to produce the same power. You can think of horsepower as torque duration. The higher the revs, the more the torque can be translated into higher horsepower. So, a smaller engine can produce similar horsepower to a larger engine by revving higher. It usually can’t match the torque generated by the higher displacement of the larger engine, though.
Also, more cylinders and a larger crank, and more moving parts usually hve more reciprocating mass, so it’s more difficult to move them quickly, and more dangerous to do so. There’s a no reason you couldn’t make a V8 run to 10K rpms, but you on’t really need to. Some race engines do, though.
I think this answers your questions. If I missed some, or you need clarification, lemme know.
GaryT, the RX8 is not turbocharged. Mazda learned from it’s experiences with the 3rd Gen RX-7, where they basically took it in the ass with warranty claims caused by exploding rotaries. Turbocharging a rotary is a dangerous business that needs to be done carefully. The Renesis in the RX8 is naturally aspirated.
Well, there’s alot of differences between a rotary and a reciprocating.
For one, to say it has “1.3 litres of displacement” is decieving. A recprocating goes through one cycle for every turn of the crank. A rotary, however, goes through three cycles for every turn of the crank. So one could make the argument that it’s “really” a 3.9 litre engine. So that’s how it makes 250 HP.
So technically it is only a 1.3 litre engine. But not really.
Ah, this is all very interesting, and thanks for all your replies. Just a couple more quick questions. How reliable would a rotary engine be? I would think that fewer moving parts = less to worry about, but then again, I’ve always been told that you’re more likely to mess stuff up if it’s going faster (higher RPMs). I’d also imagine that finding someone to work on a rotary engine would be a bit tougher than finding someone to work on a standard engine.
And finally, why don’t we see rotary engines more often? They don’t seem to have many drawbacks, but I guess there really aren’t many advantages either. Did Mazda decide to put in the rotary just for the heck of it, just to be different? I really don’t see a compelling reason to choose a rotary engine over a standard V6 or 4cyl turbo.
Well, the reliability of the Renesis engine is as of yet undetermined, since they havn’t been around that long. Reliability of the old twin-rotor tuboed RX7 rotaries was not so good, though. But, with rebuilds being mega-cheap, it’s not really a big deal if you have to do one every 60K miles.
My guess: The Renesis will have fair to middling reliability. Not Honda or Toyota like, but not bad. Just a guess, though.
As far as finding people to work on them, yes, it’s more difficult. You don’t want to buy a rotary if you’re stuck in a small town far away from everything, and with only one mechanic near you. Or, you could learn to work on it yourself.
We don’t see rotary engines more often because they have lots and lots of issues that no one was willing to spend the time to solve. Issues with running rich, and failing emissions, issues with blown apex seals, leaky seals, massive oil use, etc. Heck, even Mazda didn’t originally want to bring back the rotary, until some of their engineers fixed a lot of problems in secret.
Advantages of a rotary engine? Very small, very light, very little reciprocating mass, very smooth (balanced), no complicated valvetrain. This means that the engine no longer becomes as much of an issue in balancing the car, and placement of the engine more rearward to help balance and handling becomes better.
Plus, I have to guess it’s a resonably cheap engine to manufacture.
