Phyxs

Reader beware: This will be a long post/discussion.

Let me start by introducing myself before I go getting into the specifics of this post. My name is Jon, I'm a 22 year old engineering student, FC owner, nut job, and I love to learn how things work and how they can be improved. I grew up tearing things apart just to find out how it worked and now that love has turned to cars, specifically rotary powered cars.

I've come up with quite a few ideas/questions that may or may not have been asked/tried/thought of before, but I'd like to consolidate some of them here to get opinions and information. Many of the questions have been posed to people in my everyday life but the concepts often escape them for various reasons so I figured who better to bring them to than my rotary loving comrades? So here it goes...

(All questions posed will be done with an understanding that limitations are loose. Assume unlimited time/money/people/resources when answering questions.)

A. What are the current limitations of a rotary engine?
1. What is the top speed obtainable?
2. What is the highest RPM's obtainable, why?
3. Production of rotary engines has been limited due to emissions and economy. What are the current standards, high end obtainable, and how can they be improved?
4. What is the durability/life span of a rotary engine(both with and without rebuilds using only original parts) and what limits this?

B. What improvements can be made to a rotary engine in terms of design?
1.What can be done to the a current rotor? (think size, weight, etc)
2. Can the shape of a rotor be changed? (entirely, think squares, circles, turbine shaped, etc)
3. Currently a rotor turns 3 revolutions for 1 turn of the eccentric shaft. Can this be changed? (Ie 1 rotor turn for 1 e shaft turn) And what limits this?
4. When adding rotors, what are the major difficulties that must be overcome?

C. Performance modifications and impact
1. Is it feasible to twin-charge a rotary powered car? Because of the inherently high RPMs would it make a significant difference? Why or why not?
2. Adding mw50(water/methanol) to an engine helps to increase the amount of compression obtainable/increase octane ratings/cool intake. When done to a rotary engine are the affects desirable as in other engines? What gains are achieved?
3. In terms of alternative fuels, can things like propane/alcohol/hydrogen be used to increase the performance of a rotary engine? Why?
4. Speaking of alternative fuels, when originally introduced the rotary engine was said to be capable of running on anything including diesel. Is this true and would what would gains could be achieved?

D. General questions
1. There are many determining factors when it comes to performance when it comes to engines. Torque(rotational force of the shaft), fuel efficiency, emissions, etc. What are some factors you can change to increase/decrease these? (think more complete ignition, ignition timing, reducing friction, etc)
2. What amount of torque is necessary to move a 2,000 pound car?
3. When increasing compression in a gas engine, what are the upper limits obtainable will still maintaining proper detonation?
4. Would a better vaporization process for the fuel increase performance exponentially or only slightly? How much fuel is absolutely necessary to keep the current engines performance?
*5. To me at least, a rotary engine makes far more sense than a traditional reciprocating piston engine. (continuous circular motion as opposed to violent reciprocating motion of pistons) If so, why does a rotary powered engine have the limitations it does and why is it more difficult to improve the performance to the same levels as a piston engine?

E. Thinking ahead
1. If you were asked to build the next revolutionary engine, what design would you use?
2. Many different methods have been used to move cars, especially recently. Would a single type be best or would a multi-powered engine make more sense?
3. Things like compressed air have been used to drive piston engines, could the same method be used to drive a rotary engine?
4. The leading cause of engine death is heat, what cooling methods can be deployed to decrease this heat so we can have higher rpms, more power, etc.?
5. Electric powered cars use electromagnets to power the car. The problem with these engines is when under load, their efficiency drops significantly. Could permanent magnets be used to achieve the same result?

I know this post has been long and I have great appreciation to all those who read it fully and help to answer these questions, both for my own personal knowledge as well as any one in the future who may ask these.

TL;DR: Many questions on how to improve a rotary and engines in general.

-Jon

Aaron Cake

iTrader: (1)

I'm just going to throw some stuff in here as I approve the topic. Primarily because I know quite a number of engineering students and it's always entertaining talking to them.


I can save you the trouble: It's all been thought about before.


Top speed of the vehicle? Depends on engine power and aerodynamics, and frictional losses.

Highest RPM of the engine depends on the physical stress limitations of the materials. From a practical point, this boils down to eccentric shaft flex and stationary gear strength. Start going past the point where the eccentric begins to flex to the side and that's where things start to fail. Don't know where this is on the Renesis, but probably around 11K. Around 10K on the earlier engines. This can be raised in the standard ways: lighter rotors, balanced rotating assembly, add a middle bearing to the eccentric.

No idea what the current emission standards are, but there is a fundamental issue with the rotary engine and meeting emission/economy standards: the combustion chamber is long and narrow. A fundamentally bad shape for thermal efficiency, and tends to trap hydrocarbons at the trailing apex. Stratified charge created by direct injection can go a long way to solve this, but it will never equal a piston engine with it's almost ideally shaped combustion chamber.

Really? Just do a bit of research. The engine is durable up until the point something brakes. And that can vary wildly just like any other mechanical system. NA engines are typically good to the 200,000+ mark, turbo engines slightly less. After a rebuild depends entirely on how worn stuff is. If within spec, it may last another 200K. If out of spec, it may fail again after 10K. The limits of the life span of the engine is wear on all friction surfaces. Come on, as an engineering student you don't know this? Primary wear points are the apex seals and housing surface. Secondary wear points are the irons.

There are many configurations. Obviously one can change rotor width. Mazda shares the same eccentricity between all production rotary (except the 13A) and just makes the rotors thicker. Look up the measurements of the 10A rotor, 12A rotor and 13B rotor and you'll see. Then just for giggles, look up the 13A.

All kinds of weird rotary configurations have been tried but it is the Wankel epitrochoid that is the most successful.

You're an engineering student and you are asking this? The basic geometry of the Wankel dictates the number of rotor revolutions the rotor must make in relation to the eccentric shaft. Hit up YouTube and look at rotary animations, paying careful attention to the shape of the eccentric shaft and how the lobe moves within the rotor bearing. The eccentric shaft shape sets the path of the rotor. Change it and everything else changes. The rotor no longer traces out an epitrochoid.



Eccentric shaft length is the major one. As the shaft length grows, more support bearings are needed to keep it from flexing. Then twist becomes an issue. Obviously we also must loose all that heat so in a large multirotor configuration, we can't just circulate coolant front to back like in a 2 rotor.

Sure. Is it worth the trouble? That's up for debate. Try a search for "twin charged FD".

See the "auxiliary injection" subforum. Charge cooling via AI works in a rotary just like a piston engine. Latent heat of vaporization and all that stuff. Fairly standard and well documented.

You can burn many fuels in the rotary. Hydrogen has a low energy density, so you won't be gaining power by burning it. Propane has been used many times in the past but you'd have to ask someone else how it works out. Alcohol has the same advantages in a rotary as it does in a piston engine (Google) though lubrication of the seals can become an issue as many oils don't mix well with alcohol.

Yes, true. Gains, advantages and disadvantages would depend on the fuel. You're not going to get high enough compression to auto ignite diesel due to the geometry of the engine limiting compression to somewhere in the 12:1 range (flame front splits into two faces after that). So I believe that rotary diesel is still spark ignited. Some Googling will turn up various rotary fuels.

Same as modding a piston engine. Reduce internal friction, improve balance, burn more air and more fuel more efficiently.

Really? Isn't this engineering 101? And of course would depend on the amount of friction in the system (is the parking brake on?) and how quickly you want to move it. Torque at the wheels? Torque at the engine?

I wouldn't say that any detonation is proper!

Isn't there a chapter "Basic Designs Of The Otto Cycle Internal Combustion Engine" in an engineering text somewhere?

The more we can vapourize fuel, the better it will burn.

How much fuel depends on a LOT of parameters. Every engine will be happier at a different air/fuel ratio and this is why so much time is spent tuning an engine. For example, an NA 13B may make the most power at 13.8:1 w/ 30 degrees of timing. Run that in a turbo 13B and you won't have an engine left to make another run. That said, theoretically you could do that if it weren't for the pesky things like combustion temperature, exhaust gas temperature, etc. There are plenty of resources on the basics of tuning gasoline engines which will cover the process.

To really bake your noodle, the rotary engine both makes far more sense and far less sense than a piston engine. While a rotary may be more mechanically efficient, it is far less thermally efficient. It is a tradeoff between lots of power in a small and light package, vs. fuel economy and emissions. With the improvements in piston engines, in the NA world at least, the gap is closing.

What is a "multi powered" engine?

Hint: Both a piston and rotary engine will work as a compressor.

Standard oil and water cooling, fluid to air. Maximize the surface area within the engine for transfer into the fluid, and maximize the surface area in the air to dissipate heat. Design the cooling passages in the engine to minimize hot spots caused by stagnation, while maintaining an ideal velocity of the fluid to assure maximum heat transfer. Run the system at a higher pressure to raise the boiling point....All standard stuff when designing such a system (there should be a chapter in that engineering text on it).

Go back to the text book and read up on how an electric motor actually works. There are few high power permanent magnet electric motors primarily because the magnets would be huge, expensive and heavy. Plus it's nice to be able to vary the speed of the motor by controlling the magnetic field.


Do you have an SAE membership? Mazda has released about a billion SAE papers which cover rotary engines in excruciating detail.

patman

iTrader: (3)

Do a search for "Rotary Engine" by Yamamoto. I don't know if you can still legally buy it, but it is available for download as a PDF several places. This will answer most of your questions, and help you to gain a much better understanding of wankel basics.