Why are there no more two-strokes?

Traditional two-stroke engines offered a variety of advantages over their four-stroke rivals.  Their power output far exceeds similar capacity four-strokes. This is in part due to the fact they produce power twice as often as a four-stroke and partly because there are less moving parts to create drag and losses in power.  This “fewer moving parts” factor was seen as another significant benefit of two-stroke technology.  Fewer moving parts equates to fewer things to go wrong.

The late 80’s and 90’s could have been considered as the high point of two-stroke motorcycles.  The premier racing category (now known as MotoGP) featured three classes of bikes featuring two-stroke engines.  A few manufacturers had small light-weight high-powered two-stroke road bikes in their line-ups.  Of course, every silver-lining has a dark cloud somewhere. With two-strokes, this cloud has a blue tinge and a distinct smell about it.  Two strokes were notorious as being bad polluters and suffering poor fuel economy.  “Highly tuned” two strokes were also known for their light-switch power delivery.  “All-or-nothing” power delivery can be intoxicating, but it can also be annoying and down-right dangerous on public roads.  When not running at optimum engine speeds, these two-strokes expel a large amount of unburnt hydrocarbons into the atmosphere.  So much so, that even before Al Gore brought climate change to mainstream attention, most people could see that this was “altogether a bad thing”.

Of course, two-stroke technology still exists in all sorts of industries today.  Outboard marine engines often feature this technology.  But, before you start criticising your boat owning neighbour for his “careless attitude toward the environment”, know that modern outboard two-stroke engines pass the emissions tests required of it.  Direct Injection technology ensures that fuel is not wasted and pushed out the exhaust unburnt as did two-strokes of old.  The fuel is only delivered to the combustion chamber when it cannot escape out the exhaust port.  (My apologies to any reader who doesn’t understand the basics of two stroke combustion engines – hopefully I’ll cover that in an introductory manner at another point in time)

From what I have read, direct injection two-stroke engine design:

  • Eliminates the “peaky” power delivery.
  • Reduces emissions to comparable levels of a four-stroke engine.
  • Retains its power to weight ratio advantage over four-stroke engine design.

I hear you saying: “Surely they have lost some of the advantages they used to have?  Isn’t there always a compromise?”.  Well, as stated earlier, two-strokes of old were mechanically very simple.  Few moving parts with very little to go wrong.  Direct injection necessitates that things get a bit more complicated, with fuel pumps, fuel injectors and so on.

I suspect the biggest factor in why we don’t see modern “clean running” two-stroke motorcycles is largely due to the sins of their past.  Any new bike would need to overcome the old stereotypes of being polluting and thirsty motorcycles.  Given that technology exists to overcome these issues, it really is a shame the manufacturers have not risen to the challenge of marketing them in a better light.  There still seems to be a lot of sentimental folks in the motorcycling press who would like to see them return, so maybe they will, one day…