Aspin and Cross Valves

Aspin_valveBy Kevin Ash


Pictures: Yamaha Press, Kevin Ash




Even though poppet valves are universal in four-stroke bike engines, regardless of the number of cylinders, layout or application, still some engineers keep looking for alternatives. Poppet valves are good in that they’re cheap to make, they deal with combustion chamber pressure very effectively (the higher the pressure, the better they seal), and they’re easy to lubricate without much oil getting into the combustion chamber.

poppet_valves_2Poppet valves with their springs and cams generate lots of internal power losses - Click on imageBut there are fundamental drawbacks, hence the quest for something else. Most obviously, even when fully open, the valve head still presents a considerable obstruction to gas flow – the inlet charge must flow around the edges, which is not very efficient. Secondly, to achieve a large valve area, the cylinder bore has to be very wide, compromising the designer's bore and stroke requirements in some types of engine. Also the reciprocating valve movement causes some vibration, presents a lot of inertia to overcome, can get in the way of piston movement and with the heavy springs that most engines need, generates a lot of internal friction.

The best known alternatives are Aspin valves and Cross valves, both of which have been tried on bike engines in the past. An Aspin valve is a conical shaped sleeve which fits inside the combustion chamber and rotates, driven from the crank by a vertical shaft. As it turns, it covers and uncovers ports much as a two-stroke piston does in cylinder walls.

The advantages are considerable: when the ports are uncovered there is no obstruction, just an unhindered hole for gas flow in and out of the combustion chamber. These were invented by Frank Aspin in the 1930s, and were subsequently developed by his engineering company over the next 40 years or so, during which time the company claimed substantial improvements over poppet valves. Aspin_valveYellow shaft on right takes drive from the crank, turning the green gear, which spins the conical Aspin valve (orange) opening and closing ports in the cylinder head - Click on imageEngines converted to Aspin valve operation (many of them motorcycle engines) apparently showed massive gains in torque, an ability to run at extremely low rpm as well as having far higher rev limits, much greater tolerance to poor quality fuels, better exhaust emissions than contemporary engines and much higher maximum power and torque outputs. A 250cc Rudge for example was said by Aspin to make 31bhp at 10,000rpm and be capable of revving to 14,000rpm! But these were never backed up by independent testing, and the design suffered major problems with seizure, oil consumption and overheating.

The Cross valve appeared more promising. This comprised a hollow tube which rotated at half crankshaft speed inside a sleeve running across the top of the cylinder. The inlet gas was fed into one end of the tube, then went into the combustion chamber when an aperture in the side of sleeve aligned with a port in the top of the chamber. Norton’s development engineer Joe Craig worked on a Cross-style valve in the 1960s on a single-cylinder Manx engine, but came up against the same problems as Aspin, notably sealing against combustion chamber pressure and oil consumption. Plug fouling was an issue and in the end the motor still made less power than a stock one.

poppet_valves_1Poppet valves are appealingly simple - Click on imageThat’s not to say these designs would never work. Advances since then in materials and sealing technology combined with highly accurate modern manufacturing and improved lubrication knowledge might overcome these issues, leaving only the advantages. That would mean more power, better economy and lower emissions. But it would take a lot of work and research, diverting valuable resources from developing systems which are known quantities, so it would take a brave manufacturer to do so.


Wed, 17/12/2008 - 09:21
#1
shuggiemac
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Very interesting article indeed, so much so that I will dig up some more research on these items to satisfy the curiosity of someone with a life time spent in mechanical engineering.
The Aspin system looks particularly interesting and I shall be interested to see how big the actual consical unit was/is. It obvioulsy had to be smallish but there is still some mass to be spinning around its own axis at pretty high speed. Do you know if it was ever tried on multi cylinder engines? I can also imagine that there would be challenges in longevity of the system and bearing arrangement as it would have to spin pretty precisely.
The Cross Valve sounds worthy of research too but again it sounds like a system where alignment and location will have to be critical as well as getting sufficient lubrication into the tube and shaft, which I guess was the problem that raised the high oil consumption.
I would have thought that it was a worthy use of resources to re-visit these ideas and let's face it the guys from the DN-01 team might be hanging around kicking their heels, as it surely is unlikely that they will be working on a model upgrade!

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