The Perfect Engine

The Perfect Engine

It does not exist. But luckily we all have different ideas of perfection.

“If you chase perfection, you often catch excellence.” William Fowble

Perfection is as hard to attain as infinity is to reach; a million miles is as nothing to infinity and perfection can only be reached by an eternity of trying.

I will make the hypothesis that just as beauty is in the eye of the beholder so can be perfection. It depends on what one thinks is important and what one can ignore. To begin, can we ignore, or at least accept, that as converters of energy stored in liquid fuel all kinds of engines are far from 100% efficient and, therefore, intrinsically far from perfect? If we can, what are the other parameters by which to gauge perfection? Some of the candidates as parameters might be the level of efficiency, specific power output, power/weight ratio, noise, exhaust emissions, size or shape, reliability/longevity, ergonomics (its feel). Does it do what was specified for it to do?

It depends upon what you mean by perfection

Do you remember the stories of the racing engine builders who could get the crankshaft and crankcase bearings so perfectly in line that it would spin under its own volition – like a perpetual motion machine? Well, that would be perfection! And I used to love it after a race to come to a stop by the van in the paddock and shut the throttle for the G50 or 7R racing engine to tick over at 200rpm. The engine was so hot there was, perhaps, a trace o automatic ignition and virtually no internal friction due to the hot, thin oil

Those engines were a sort of perfection – to me – at the time.

Jim Boughen, who had worked with my father developing the 7R AJS and G50 Matchless, built superb engines (albeit perhaps not quite perpetual motion) for the bikes that I rode for Tom Arter and when the engine did that I thought, “Perfect”. It depended, too, on getting the air jet and float chamber perfectly adjusted. The twist grip had not the slightest bit of slack which also contributed to a perfect bump start. In those days there were no clutch starts with running engines. I would push with all my might on the handlebars with the front brake on; crack the throttle open the precise amount and as the flag dropped, brake off, three steps, drop clutch, , engine fires, leap onto the saddle and first away!

Similarly, to make a perfect engine Jim had to be vigilant that all the parts and his careful assembly were perfect. In those days wider machined tolerances than today’s digitally controlled and checked machine tools can achieve, his ‘care’ was to choose parts that went together with the right clearances or interference fits. I remember the first 7R AJS engine of his I used. I called it a ‘little 500’ because it was so quick. When he finished building an engine he would turn it over using his timing disc on the crankshaft. This engine would not go over TDC and he thought that he must

have made a mistake with the piston squish clearance. Upon removing the cylinder head he found all was well and after refitting the head he simply gave the crank a little more effort than usual to turn it over. This particular engine had a particularly good compression. The roundness of the cylinder as assembled, the fit of the piston and rings in the cylinder bore were perfect. That was the critical difference.

Similarly, the poppet valves and valve seats had a radius instead of a conical shape with a virtual line contact, gas tight seal of the combustion chamber and it is only with perfect alignment of the valve seat cutter with the valve guide and precise concentricity of the valve seat and stem that this can be achieved. The clearance of the valve stem in its guide must be close to sticking to give the valve head a chance of a perfect landing after letting gas in or out of the combustion chamber. A racing engine running at 10,000rpm may have its valves open for 5 milli-seconds– so there isn’t much time to waste for the valve to shuffle about on its seat!

Digressing for a moment – thinking about that time of 5 milli-seconds. Even in these days of the atomic clock, nano-engineeing and quantum mechanics (whose recent theory is that the electrons of the atoms in our bodies are simultaneously in us and at the other side of the universe) the workings of something as relatively mundane and old-fashioned as the internal combustion is incredible and impossible to comprehend. As an example of limit of human observation and, therefore, understanding, I designed a research engine with a glass cylinder barrel about twenty years ago so that you could see the piston and valves going up and down; except that you couldn’t because they are moving too quickly for the human eye to follow, even at tick-over speed, and even at 200rpm. It is why it is o fascinating to watch the slow action replays of activity in nature, science, engineering or sport taken by the camera whose eye can see – record things that happen at high speed. Actually, that is what the scientist or engineer does at a leisurely human pace when calculating how metals and fluids behave at high speeds in very short time frames.

One of the most important features for the performance of an engine is the design of the inlet port but unless you have gas flow equipment and many, many hours of time available to you, what you have is what you use. Jim Boughen once added 15mph to the top speed of an early Norton Commando because he knew how to perfect the shape from standard inlet ports.

Perfection is, indeed, hard to define and the closest we can come is that the best in a particular period is the perfection at that time. Perhaps we would do better to ask, “What is the optimum engine”. We would still be in trouble to define it but to reach that goal, whatever it is, engineering has developed better machine tools and techniques for making things. Camshaft design and manufacture is better now so that valves ‘floating’ out of control is almost unheard of even at higher engine speed than of yore; cylinder bores are more perfectly round; pistons and rings seal the combustion chamber more nearly absolutely; oil resists the engine’s rotation less.

I think it is a credit to the designers of the engines of the ‘Classic’ era that the engines are still so conducive to perhaps an eternity of development to approach perfection.