Remember those noisy motorbikes with plumes of smoke pouring behind them as they tore down the street? I grew up with them and aspired for one, although in some respects they were death-traps for teenagers. You don’t see many two-stroke engines on the road today; they have survived in lawnmowers, chainsaws and hedge-trimmers. Cheap, light and compact makes for a good garden appliance.
A two-stroke engine applies the KISS (Keep it simple Stupid) principle with panache, so called as it had two strokes per cycle (instead of the standard four for the engine which powers our cars); it cost less than half as much to make, is far lighter and puts out far more power per revolution (firing once rather than every other revolution).
The two stroke engine performs the same four separate processes as a four-stroke (‘suck’, ‘squeeze’, ‘bang’ and ‘blow’). But it ensures the exhaust stroke (‘blow’) and the induction stroke (‘suck’) happen simultaneously when the piston is travelling through the bottom half of the cylinder. The other two strokes – compression (‘squeeze’) and combustion (‘bang’) are carried out sequentially while the piston is in the cylinder’s upper half.
Such simplicity does have a price. The fact that the inlet and outlet ports are open simultaneously means getting rid of burned gases before fresh fuel is admitted is not overly effective, meaning poor fuel economy. But the killer stroke (as it were) is its total loss lubrication system. Lubricant is pre-mixed with fuel and because oil combusts less than petrol, as much as a third escapes into the atmosphere as unburned hydrocarbons and soot. However, these two reasons are not the only reason for the two-strokes disappearance. Motorbike makers simply wanted to focus on the more profitable four-stroke models.
Two-strokes are now back in fashion. The pilotless planes such as the Predator and Reaper in Iraq and Afghanistan are extraordinarily successful examples ranging in size from 10cc to 200cc. Aircooled and running on petrol, diesel, avgas or jet-fuel.
On the road; two potentially big comebacks
The Omnivore engine (produced by Lotus Engineering) provides for direct injection and variable compression. This operates like a diesel using heat from the compressed gases to ignite the mixture (rather than with spark plugs) running on a range of fuels with high efficiency and low emissions of carbon monoxide. This provides superb competition for four stroke petrol engines which suffer from significant throttling losses when they are driven at less than flat out. Overall efficiency of a four-stroke car when muddling along the road at half throttle is typically 17% rather than 30% (at full throttle).
The Opposed Piston Opposed Cylinder (OPOC) developed by EcoMotors, where the mixture (diesel or petrol) is compressed between two pistons moving in opposite directions (thus half the distance to travel), allowing twice the rotation speed as an engine with fixed cylinder heads and providing 30% better fuel economy than a conventional four-stroke. Don Runkle (CEO) remarked (arrogantly or supremely confidently?): ‘The OPOC, is cheaper, better, simpler, stronger, lighter and cleaner than any other power generating technology now or in the foreseeable future’.
We may find these engines end up giving four strokes and indeed, electric cars, considerable competition in the next decade.
Thanks to The Economist for references.
As E.F. Schumacher noted:
Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction.
Yours in engineering learning