2-stroke engines – a revival?



 I love them because of their simplicity and ruggedness. The invention and later modifications can be credited to Dugald Clerk, Joseph Day and Frederick Cock.  Dugald Clerk has the patent rights on an engine which has a separate charging cylinder, Day saw the merits of the scavenging effect in the crankcase and Cock was the genius behind the piston controlled inlet port. Because the petrol and lubrication oil are mixed these engines can be run at any angle – model aircraft make extensive use of these engines, simple and cheap. 

As the four stroke engine has to complete two full rotations of the crankshaft to run through the suction or inlet, compression, power and exhaust cycles to perform any useful function the two-stroke powers and exhausts on the downward stroke and provides suction and compression on the upward stroke. The two-stoke also inducts in the crankcase, poor starting often attributed to crankcase wear. These smaller engines also use a rotary valve (for better torque and power range control) or a reed valve which along with piston port control makes the power band wider and more useable. The rotary valve uses a portion of the crankcase in the control process and any wear would render the entire operation useless. There are other control types used in larger engines, such as marine diesels but they are out of the scope of this article. What is important though is that two stroke engines, particularly the reed variety can run backwards. I remember a story my father used to tell me about a certain model of motorcycle (i cannot remember the manufacturer) had an engine which was particularly prone to running backwards after a backfire, normally as the rider came cruising to a halt at a stop sign, clutch in. These engines did not warn the rider that they were now running in  reverse (no mechnical tachometer on old bikes) so you can imagine the rider’s surprise when his bike then went into reverse after letting the clutch out at when pulling away. I don’t really know of many manufacturers pre-60’s that made two-stroke bikes but I do know a friend of mine’s son stripped his engine only to find it would only run backwards when re-assembled.

Problems with two strokes:

  • As two strokes do not have a wet sump lubrication can prove to be a problem at low engine speeds especially in high performance engines which would be tuned to a leaner mixture with a higher fuel oil ratio.
  • Without exhaust controlled valves a highly tuned engine relies on extensive modification of the exhaust system which can be excessively noisy, by all means higher than current road ordinances allow.
  • Two stroke engines run very erratically at low revs.
  • Crank sealing. When I used to play with these engines years back this seemed to be the biggest problem. And the most overlooked.
  • Low fuel efficiency.

Valve controlled exhaust ports

Standard two strokes are notorious gas guzzlers as they are not efficient at utilising all the fuel through the intake process for the power stroke – some fuel is lost in the process of inducting and exhausting where at the bottom dead centre portion of the piston stroke some fuel-air mix escapes through the exhaust port. Engines running at high speed are more efficient in this process because of piston crown design but there is still loss. Manufacturers have now all produced exhaust valve controlling which varies the open duration of the port. This valve makes the engine more efficient at lower revs, produced more power at lower revs and makes the idling far more smooth than without. The two stroke engine is still a marvel, it has an incredible power to weight ratio and multi-cylinder motorcycles were the fastest on our planet in the 60s and 70s. 

Where do we stand?

Manufacturers are still spending vast sums on R&D in the modification of two stroke engines and some claim to be bringing in engines with better fuel economy than four strokes. Electronic valve control is becoming prettyy much an indstry standard and there are some designs which now incorporate a wet sump. Lotus developed a monoblock two stoke having just that. Two stroke enginesnow get built without a cylinder head gasket – molding the cylinder head and cylinder as one. See Lotus Unveils New Omnivore Flex-Fuel Engine Concept and Chrysler’s Neon http://www.allpar.com/neon/stroke.html. Personally I think the days of the four stroke engine may be short lived – electronics, fuel efficiency, power to weight ratios are all things one must take ito account. I was in the merchant marine a good few years back and as one chief engineer told me with regards to steam turbines: “why make something go up and down to make something else go round and round”. Never a truer word said!

With electronics playing a vital role in all things mechanical the two stroke will not be phased out. In third world countries especially the two stroke is in most cases more viable because of their simplicity and cheapness to service. Two strokes may no longer be a commodity that becomes a throway item after a very short lifespan – I’m sure that manufacturers like Chrysler will make sure of this. 

 

 

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