We have had piston and poppet valves longer than we have had the internal combustion engine. But over the years, there has some been some different ideas and engine designs that have tried to challenge the traditional piston engine… All have failed.
What brought on this post was the recent announcement from Mazda that they were going to cease production of the RX-8 citing “falling sales and stringent global emissions standards”. So it looks like once again, the rotary has gone out of production with the same excuse as with the RX-7.
I must say that I’m really not a big fan of the rotary. I even owned an RX-7 at one point so it’s not like I don’t have any experience with the engine. The design is a novel concept, but I feel its negatives outweigh its positives and I would be shocked if the engine ever became more than a novelty that could really be a viable alternative to a conventional piston engine.
If we exclude all of the other negatives of the engine (i.e. inefficient combustion chambers resulting in poor fuel efficiency, carbon buildup problems, no variable port timing, no low rpm torque) and focus on the seals of the engine, we find the issue that seemingly all alternative engine designs face. Unlike a piston engine which has a round bore which is sealed almost naturally with a piston ring, rotary and other alternative engines require complicated apex, corner and side seals to seal off the combustion chamber. Apex seals on a rotary engine also experience combustion loading on both sides of the seal which further complicates matters.
Lubricating these seals is also an issue and has been a problem in rotary engines since their inception. Generally, rotary engines have had some sort of oil injector which squirts a little engine oil onto the sealing surfaces. In older engines (pre RX-8), these systems were prone to failure and the only truly reliable way to assure seal lubrication was to pre-mix your fuel with 2-stroke oil. From what I hear, the RX-8 system is much more reliable than the old RX-7 systems, but, this brings up another issue… These seals are sensitive to the type of oil used. If you own a rotary engine, you have probably learned that you should never run synthetic oil in your rotary. The reason is because synthetics are more resistant to burning off and can buildup inside the engine. This buildup becomes large over time and can lead to seal failure.
For the most part, these issues may never become a problem in the RX-8 if the engine is left stock and run Mazda’s recommended oil. But, there does seem to be a lot of engines failing since there must be reason why Mazda issued an optional recall on the RX-8’s engine.
I’m really ranting on the Wankel engine for the most part, but these problems are shared with any engine that doesn’t use pistons in cylinders or has strangely shaped surfaces that need sealing. Other issues that have prevented alternative designs from working are problems such as oil evacuation, proper cooling, and poor intake and exhaust efficiency due to strangely shaped intake/exhaust manifolds.
Another area that has been attempted to be improved upon with little success is the poppet valves on a conventional engine. The problem with poppet valves is that the incoming air does not have a direct line of sight into the cylinder; and once that air has reached the cylinder, it still has to make it around the big end of the valve. Other issues with poppet valves are they are difficult to actuate at high rpm’s, and the exhaust valve causes a hot spot in the combustion chamber. But the beauty of the poppet valve is that combustion pressure helps to seal the valve against its seat, and because they are round, they heat and expand evenly. They also don’t need any lubrication where the valve meets its seat since they are not sliding.
Many alternatives that have been proposed use some type of rotary valve that open and close ports as they spin. One company proposed a promising design was Coates. Their design uses a play off a ball valve that replaces all traditional valve hardware. The system seems very ingenious until you start to think about the forces it has to deal with. Unlike a poppet valve which seals tighter under pressure, the opposite is true for the Coates valve since it’s on top of the cylinder outside the combustion chamber. This means that all of the force from combustion is trying to blow the valve off the top of engine, not push it against a seal like the poppet valve. Coates also claims that their ceramic seal doesn’t require lubrication; since this valve has never been released to the public for 3rd party evaluation, I have my doubts on this. But I’m sure like many other attempted solutions, their design does work – at least for a little while.
So for the most part, it seems like the major problem with alternate engines (and valve) designs is that they are difficult to seal. Many people think that it’s only a matter of time before technology is good enough to figure out these problems, but I’m not so sure. I think in a lot of these cases, physics will have to be defied.