Dirty Little Secrets of Two-Stroke Engines

The Paradox of Two-Stroke Efficiency & Emissions

While small carbureted two-strokes are notorious for unburned fuel short-circuiting, large direct-injected two-stroke diesels power the world's most thermal-efficient marine vessels.

The secret that everyone knows is that two-stroke engines are dirty. They are gas hogs, noisy, and smoke more than a burning tire. Because of increasing concern over fuel consumption and emissions, the sales of new two-stroke powered equipment are being banned in country after country for all except the smallest applications. Even four-stroke powered grass trimmers are starting to show up. So what's so secret?

Actually, the secret is that two-stroke engines are not inherently dirty. In fact, the cleanest, most efficient engines in the world are two-stroke engines. Diesel-burning power plants, locomotives, large trucks, and ocean-going ships are often powered by very large, low-speed two-stroke engines. Some of these engines are so large you can actually stand inside the combustion chamber. The largest engines in the world are two-strokes, and so are the smallest. Two-stroke engines represent both the cleanest and the dirtiest engines in existence.

What makes the difference between a clean and dirty two-stroke engine is when and where the fuel is added, and there are big differences between the big, efficient two-strokes running our ships and the dirty little ones we have trimming the grass and moving our motorcycles about. Small engines are very simple devices, making them inexpensive and lighter weight than similar displacement four-stroke engines. One of the biggest differences you'll notice on a small two-stroke engine is the lack of valves in the head. Generally, the head houses only a spark plug and some cooling fins. After combustion, hot exhaust gases push the piston down in the cylinder, exiting via an exhaust port in the cylinder wall about halfway down the stroke. Because the piston is moving near maximum velocity when uncovering the port, it opens rapidly. This creates a large exhaust pressure pulse that propagates into the pipe, giving the two-stroke engine its characteristic sharp sound. In a four-stroke engine, the exhaust valve lifts gradually off its seat, resulting in a slower rise in exhaust pressure and a less harsh sound.

As the piston continues descending in a small two-stroke engine, it uncovers transfer ports in the cylinder wall admitting fresh air and fuel from the crankcase. As the piston descends towards bottom dead center, gases in the crankcase are compressed and push their way into the combustion chamber through the transfer ports, sweeping remaining exhaust gases out through the exhaust port. As the piston rises again, transfer ports close first, followed by the exhaust port. Inevitably, some exhaust gases remain, while unburned air and fuel escape out the exhaust. This "short-circuiting" of air and fuel out the exhaust port is the root cause of high fuel consumption and smoke in simple two-stroke engines.

Were it not for this fuel short-circuiting, the two-stroke engine could be inherently more efficient than its four-stroke counterpart. Four-stroke engines must drive a camshaft and valvetrain (consuming mechanical power) and rotate two full revolutions per power stroke (doubling friction losses). Furthermore, four-stroke pistons require dedicated oil scraper rings that account for significant mechanical friction, whereas two-stroke pistons typically use only 1 or 2 thin compression rings.

So, how do we keep fuel from escaping out the exhaust port? Direct Fuel Injection (DI) is the solution. Fuel is sprayed directly into the combustion chamber once the exhaust port is sealed. High-pressure air-assisted or 100-bar hydraulic direct injection systems are commonly used in clean, high-efficiency two-stroke engines found in European 50cc scooters and outboard marine engines.

Larger two-stroke engines are all direct injection systems burning natural gas, diesel, or heavy fuel oil. In large marine engines, overhead exhaust valves handle exhaust while cylinder wall transfer ports supply pressurized air from turbochargers or superchargers rather than crankcase compression. Operating at low speeds dramatically reduces mechanical friction and allows ample time for complete combustion, enabling large two-stroke diesels to routinely achieve thermal efficiencies exceeding 50% (compared to ~30% max for automotive gasoline engines).

Research at Focus Applied Technologies and Universiti Sains Malaysia (USM) focuses on developing next-generation direct-injected two-stroke engines, paving the way for ultra-efficient small diesel powertrains capable of achieving over 100 km per liter on alternative biofuels like used cooking oil.