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Two Stroke Tipple

fuel-lubricantsLove them or hate them, the two-stroke engine is in many ways far superior to its 4-stroke rival.

Specific power, specific weight, manoeuvrability, cost of manufacture, ease of maintenance, durability, NOx emissions and even fuel consumption in the case of smaller engines, can be far superior to its less controversial 4-stroke brother, but why then don’t we see more of these engines in competition?

With one strike for power and one to wear it out as opposed to one of power and three to wear it out for the 4-stroke, used in most forms of karting and in some smaller engined motorcycle racing, the basic two-stroke engine can appear visibly dirty to the casual observer. High amounts of toxic unburned fuel and an occasional slight tinge to the exhaust suggest a high level of particulate matter when running on traditional gasoline fuels. But if that fuel were changed to a more environmentally benign ethanol or blend of ethanol, then there is increasing evidence to suggest that not only will these objections be overcome but that when fully optimised, two stroke engines can produce even more performance.

Essentially, it is all about charge density. In a 4-stroke naturally aspirated engine, any benefit given up by the fuel is mainly due to its increased octane number or resistance to detonation. This allows an increase in compression ratio and more efficient combustion. In a two-stroke engine however, not only can we use the increased octane available to create more efficient combustion, but the process whereby the intake charge is compressed in the hot crankcase can be used to extract the maximum benefit out of the latent heat of evaporation which at 904 kJ/kg is something like 2½ - 3 times that of gasoline. Not only that, but a greater cooling effect in the crankcase produces an increased charge density, which in turn expels a greater pulse of gas into the exhaust pipe. This greater pulse is consequently reflected back and used to pull in even more intake charge into the cylinders. And since the performance of a two-stroke is all about the exhaust system and these pulse-tuning effects, the effect on the performance of the engine using ethanol is significant, to put it mildly.

In blind back-to-back testing with gasoline on a 125 cc kart, after doing a few modifications to the engine and changing to a blend of ethanol, the driver reported a more responsive engine with much better pulling power from a lower engine speed. At the end of the session asked if he wanted to revert to the earlier specification his comments were un-publishable but needless to say the offer was refused. And while running rich, the amounts of carbon monoxide produced will most probably be of the same magnitude to that of gasoline, there is some evidence to suggest that the un-burned hydrocarbons typical in a basic 2-stroke, may be less toxic.

It’s not all good news, however. The corrosive effect of alcohol fuels on brass or aluminium in the carburettor or engine crankcase has to be carefully managed. As well as running on regular gasoline for a period at the end of the day and regular changes in crankcase lube, consideration has to be made on the type of lube oil added to the fuel. While many of the more usual two-stroke oils may not be suitable, those containing high levels of esters are claimed to be much more miscible.

With thermal efficiencies approaching 55% in some types of sophisticated 2-stroke designs, rather than trying to ban them in motorsport, shouldn’t the authorities be trying to encourage the use of more sophisticated versions? A change to a more environmentally benign fuel such as ethanol, could therefore just be the starting point.

Written by John Coxon.

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