Why a Two-Stroke Engine?

What comes to mind when you think of a two-stroke engine: a moped, scooter or chain saw? Or, maybe it’s the sight or smell of blue smoke. The Achates Power opposed-piston, two-stroke (OP2S) engine bears no resemblance to these traditional two-strokes. For one, in our engine, the fuel and oil don’t mix, so there’s no smoke from burning oil. And, since the fuel is directly injected into the combustion chamber, our engine doesn’t suffer from high hydrocarbon emissions like its two-stroke counterparts. But, these aren’t the only reasons opposed-piston, two-stroke engines are garnering interest. The two-stroke engine has potential thermodynamic, package and economic benefits that are hard to ignore.
 
As we detailed in our 2011 technical paper, the OP2S engine has inherent thermodynamic benefits, a portion of which result from the opposed-piston architecture (a topic for another blog post), and a portion of which result from the two-stroke operation. There are two distinct advantages associated with two-stroke operation that lead to greater work extraction for each unit of fuel energy input. First, since each cylinder fires on each engine revolution, about half as much fuel is injected for each combustion event. At the same boost levels (that is, the amount of pumping work applied to get fresh air into the combustion chamber), the ratio of air to fuel is higher in the two-stroke engine—the amount of fuel is halved, but the amount of fresh air is not quite halved. This results in a leaner combustion or, more technically, a more favorable ratio of specific heats (see Wikipedia for more information), and a higher indicated thermal efficiency. Of course, the designer of the engine may choose to alter the boost levels of the two-stroke engine in order to reduce pumping losses, but would only do so if this further increases the overall efficiency of the two-stroke engine.
 
Second, because of the reduced fuel mass per combustion event of the two-stroke, the combustion duration is shorter and leads to great work extraction. Most engines have noise constraints—they have to be quiet enough to comply with environmental regulations or market demands. One of the major causes of engine noises is the maximum pressure rise rate (MPRR)—how quickly the pressure increases in the combustion chamber. For a passenger vehicle, for example, a typical limit for the MPRR is 5 bar per crankshaft degree. MPRR, along with power, torque and emissions considerations, are constraints for engine calibration. Since each combustion event of a two-stroke engine has roughly half as much fuel as the four-stroke counterpart, the fuel can combust more quickly without exceeding the maximum pressure rise rate limits, requiring fewer calibration tradeoffs.
 
In addition to the inherent thermodynamic benefits, the sizing of the two-stroke engine provides a design tradeoff that can be taken advantage of. Because the two-stroke engine fires once a revolution per cylinder, the two-stroke engine size can be selected such that it not only has a greater power density than the four-stroke engine that it is replacing, but it also has a lower brake mean-effective pressure (BMEP). The tradeoff between BMEP and displacement is shown below.
 
The lower BMEP levels lead directly to lower heat transfer losses, which helps the engine efficiency, and to reduced in-cylinder NOx emission formation, which results in still fewer calibration tradeoffs.
 

In addition, the decreased engine displacement leads to reduced engine package size, weight and cost, all of which are favorable for engine programs.
 
There are many reasons why two-stroke engines are staging a comeback for transportation. If one set out to design the most optimally efficient engine, one key attribute of the engine is that it would be a two-stroke. In order to fully utilize the two-stroke engine advantages, however, the engine must employ an opposed-piston architecture, the reasons for which are the subject of a future blog post.