Exhaust Port relocation ideas.

[NoDOHC]

Ok, so I have hinted around for a really long time, now I finally got a moment to post this for discussion.

Simple explanation:
Exhaust port opens too early, hurting thermodynamic efficiency, increasing EGT, and exhaust sound from blowdown.
Intended Fix - Machine exhaust port in different location on RX8 housing (weld coolant passages and fill coolant seal grooves).

Complicated Explanation:
Although a full stroke on a rotary engine requires 270 degrees of eccentric shaft rotation to complete, the time that a port is open is the same as a piston engine given the same speed and port timing.

Unlike a piston engine, A rotary engine exhaust port completely opens in about 25 degrees of eccentric shaft rotation and remains completely open until about 45 eccectric shaft degrees before completely closing (due to a different position of the rotor).

A normal cam for a piston engine opens the exhaust valve at about 45 degrees BBDC. The opening really means leaving the seat. The valve is barely open by BDC. On a rotary engine, the exhaust port is completely open by 50 degrees BBDC. This means a lot of blowdown and wasted energy (any loss of gas pressure in the chamber BBDC is a loss of useful work that the gas could have done). A wild (barely idles) piston engine camshaft will open the exhaust valve at 75 degrees BBDC, but once again, the valve doesn't completely open until 40-50 degrees ABDC. These engines are not typically noted for their stellar fuel economy either.

Anyway, the point that I am trying to make is that the rotary engine efficiency problem may very well be linked to the early exhaust port opening.

Description of proposed solution:

This is the position of the rotor in the chamber when the factory exhaust port opens. As can easily be seen, there is a significant amount of chamber volume remaining when this port opens (about 18%).


This is the position of the rotor at maximum chamber volume. (What I have been calling BDC).


This is the position of the rotor at the proposed exhaust port opening.


This is a closeup of the rotor when the eccentric shaft is 230 degrees ATDC.


A mockup of how the exhaust port sleeve (which would have to be modified) could go in.



This is a comparison between old exhaust port and new proposed exhaust port.



These pictures illustrate that a set of RX7 headers could be easily bolted to the RX8 housings.

The exhaust port would also close later, but it is almost completely shrouded by the rotor at that point, so the little bit of port overlap caused by this shouldn't be an issue.

What do you guys think?