I've been pondering the efficiencies and environmental impact (or lack thereof) of diesel/biodiesel/etc.

It occurred to me that while the compression ratio on a 13B is nowhere near high enough to pop off a diesel charge, the bulk of the combustion chamber volume is in the bathtubs. If those are filled in, the compression ratio will go up significantly.

Has anyone done the math about how high you could get the compression with a flat bathtub (no dip at all)? If it's able to be pushed into the 20:1 range, then a diesel rotary should be possible using normal 13B parts.

Fill in the bathtubs, replace the leading plugs with fuel injectors, and possibly (if needed) add a set of glow plugs.

It should be possible to tweak the stock ignition system to modulate the injectors (mechanical "amount injected" linkage, electronic "inject" signal based on spark plug signals).

Is this something that seems within the realm of possibility? I'm going to have plenty of time this summer to work on interesting projects, and I think a diesel RX-7 would be pretty far up there in terms of "neat work" - it should turbo quite nicely as well.

Thoughts?

-=Russ=-

 

Damn !!! I'd be interested to see that actually !! An envirementally friendly RX-7 ! lol Nice idea !

 

Ehm... just found the thread down the page.

Eep. Sorry.

-=Russ=-

 

Reading the other thread, it looks like filling in the bathtubs too much causes problems because the intake charge can't really get in. Though that may be more on peripheral ports and less on side ports.

Another thought mentioned is the use of a turbo or supercharger to raise the compression significantly. This would allow the use of unmodified (or minorly modified) rotors - if you're running 9.7:1 rotors, a 15psi boost should raise compression to diesel levels. Starting would be a trick, but glow plugs might be able to get you going. Just size the supercharger such that it'd be constantly putting out @15psi, and go - you don't have throttle plates, so sizing the supercharger to keep constant boost would be a lot easier.

*ponders*

Other thoughts?

-=Russ=-

 

Apex seals? are there any at all on the market that would handle the shock? I think that would be a bigger hurtle than the actual fuel conversion.

 

Diesel is a "softer" combustion than gasoline. Contrary to popular belief, it is NOT detonation - combustion pressures are higher, but a 150-200hp diesel should be pushing lower peak combustion pressures than a 400-500hp gasoline rotary.

-=Russ=-

 

huh you're right. I was just doing some reading. Diesel burns at a slower but more constant rate than gasoline. Gas gives a fast impulse while diesel pushes at a pretty constant rate for a longer duration. The long rotary combustion phase would really lend itself to the long diesel burn.
I'm envisioning a 12A with 2 VW mechanical injection pumps.

Was searching a bit, interesting info:
https://www.rx7club.com/rotary-car-performance-77/rotary-diesel-538892/

 

You can get in the area of 13:1 compression on a 13B. It will not affect intake but it will greatly affect flame front propagation on a gas engine.

 

Does that have any depression at all? I would tend to think you would need some depression to hold the compressed fuel/air mixture. Even at 13 to 1. Out of just plain curiosity and the fact that I've tried welding rotors (to fix apex seal grooves), What type of rod/material did you weld with?

 

this rotor is flush. Filled with stainless steel and below the stainless is a special coating that we use as a bond agent. The rotor is CNC milled with our own depression design or to a custom design if requested. A gas engine would NOT run properly in this configuration.

 

Well, that answers that. If completely filling in the depression will get me to ~13:1, I'll definitely need a blower on the front end to get compression to diesel auto-ignition temperatures.

How much do those flush rotors cost?

Thanks!

-=Russ=-

 

Here is a picture of a true wankel diesel engine. It is direct injected and uses a glowplug. It is not spark ignited. This is an apu (auxiliary power unit) for business jets built by the PATS company. Scroll down a little bit.

http://cp_www.tripod.com/rotary/pg08.htm

 

IIRC the depression helps to allow the flame to travel. Because of how the rotary compresses the air/fuel mixture gets pinned in a corner. The bowl lets some of that mixture sit in it to burn, and gives the flame front an area of travel when the leading edge of the rotor face starts expanding away from the housing wall. Since compression will be higher at that point on the trailing side of the rotor the mixture rushes towards the expanding side. Higher --> Lower pressure.


BC

 

I think I see a combustion chamber in the one pic.

 

You do. It's not direct diesel injection but rather indirect injection through the use of a subchamber. Although this is seen as a step backwards in diesel technology compared to the super high fuel pressures they run now, it is easier on the seals as the initial shock wave from combustion isn't a harsh. It has it's advantages and disadvantages. Remember that as an apu, it is probably optimized to run at one rpm rather than over a range.