The direct-injection 2-stroke engine - all the advantages of a two-stroke, none of the disadvantages

by | Nov 8, 2010 09:25 AM ET

Last week we talked about gasoline direct injection engines that are starting to appear in cars from every major manufacturer. These engines get better gas mileage by allowing higher compression ratios and leaner mixtures.

A reader named Chris wrote in and suggested I check out Evinrude's E-Tec engines. These use direct injection technology on 2-stroke engines.

Two-stroke engines have two big advantages over four-stroke engines:

  • Two-stroke engines are much lighter than four stroke engines of the same power, and...
  • 2-stroke engines are much simpler, usually meaning less maintenance.

Both advantages come from the fact that there is no valve train in a two-stroke engine, meaning no camshaft, no valves, no valve springs, no timing chain/belt, and so on. The problem with 2-stroke engines has been lots of oily pollution and low efficiency (2-strokes burn more gas than 4-strokes of the same power).

The advantage of a direct-injection 2-stroke engine would be the greatly reduced weight of the 2-stroke design, while at the same time eliminating all the pollution produced by a carbureted 2-stroke and increasing the efficiency. This video demonstrates where the pollution of a carbureted 2-stroke engine comes from, and how direct injection eliminates it:

The following video describes some of the advantages of the direct injection approach in 2-stroke engine. The most interesting thing is the weight comparison - a 150 HP direct injection 2-stroke weighs 70 pounds less than a 150 HP 4-stroke (about 15% less):

So the question is, how is a direct-injected 2-stroke engine lubricated? It can't use a sump system like a 4-stroke because the crankcase is still acting like an air compressor. And you can't mix the oil with the gas as in a traditional 2-stoke, since the gas never makes it into the crankcase. An E-TEC engine uses an external oil tank that holds about 2 liters of oil - enough for about 40 hours of engine operation. The oil is slowly injected to places like the crank bearings and cylinder wall. Therefore, the oil does mix with the air getting pumped through the crankcase. It gets burned in the combustion process. The amount of oil is so small that it has no noticeable effect on emissions, and it has none of the pass-through problems with oil as in a carbureted 2-stroke. Also note that, by burning the oil, there are advantages. There is no pollution potential when changing the crankcase oil of a 4-stroke engine (e.g. person spills or dumps oil on ground or water), and there is no possibility of forgetting to change the oil and ruining the engine.

This article offers some additional insights on the engine. For example:

The E-TEC's electrical system is based around a magneto -- like a Model T -- for a simple reason: recreational vehicles are put into storage at the end of each season, then pulled out when the weather turns favorable. Batteries die in storage, so relying on one to drive the fuel injection system and engine controller only adds to customer frustration. "The magneto produces from 150 to 300 volts," says Broughton, "but that is reduced to 55 volts to drive the oil and fuel pumps and the fuel injectors. It's further reduced to 14.7 volts to charge the battery, if the boat has one.

More info:

- Two-stroke engines

- Four-stroke engines

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