So I was talking to a buddy of mine about general engine theory. He asked me an interesting question that I really didn't have an answer for

"The dishes on the side of the rotor face, if you made those smaller wouldn't you increase compression?"

If thats true how come you don't see shops offering this service? Or for that matter more custom diesel rotary engines seeing as how you could effect the compression rate?

What are they actually called/technical name? I couldn't really find any information about them.

 

You mean the bath-tub shaped combustion area?

You would have to fill in material. Not exactly an easy thing to do.

 

you would not increaes compression because the dish is there during compression stroke as well as intake. i.e. no matter were the engine is there is always that volume being added in. The purpose of the dish is to help with combustion, the odd shape of the rotary engine makes it hard for a smooth even flame inside, the dish along with having 2 sparkplugs per combustion chamber helps burn all the fuel.

 

Your theory is wrong. That would also apply to piston motors with domed and dished pistons. The sweep volume will change slightly but when compressed into the combustion area the ratio will change. Rotor or pistons the math is the same.

Whats the difference in a turbo'ed and non-turboed rotor? I know they are interchangeable but that does not mean they are the same. Those dishes might be different, I just haven't found any information on any of this online.

 

Changing the dish size does alter compression ratio. Dish size is the only difference effecting compression ratio of all the 13B engines.

To answer the original posters questions, you do see shops altering the shape and size of the dish to alter compression and quench characteristics. Mazda has also used many different compression ratios and chamber shapes over the years. E&Js aluminum rotors use a unique shape for the dish and it seems to be working well for them. Their plans are to build some 13-14:1 rotors for use in NA race engines. Conventional theory says compression ratios higher than 10:1 won't work well in a rotary but E&J is convinced they can make it work.

 

Mazda has old charts showing that power from pumping losses falls off above 11:1 compression. The reason is that unlike a piston engine, a rotary doesn't just compress air in one place but also flows it through the engine. All of the air passes through the dishes and if it is too small the air can't flow efficiently causing pumping losses.

 

It would be interesting to see the rotor geometry of these new 16X engines, especially the spark ignited diesel prototypes that Mazda is working on.