Types of 4-Stroke Internal Combustion Engines


A typical four-stroke engine or an Otto cycle engine does intake, compression, combustion, and exhaust. The Atkinson cycle and the Miller cycle engines also do intake,  compression, combustion, and exhaust, however, they differ in a small but very important detail which allows them to be significantly more efficient than the Otto combustion cycle. And in this video, we will dive back into the history of engines in order to see and learn about the evolution of the combustion engine and the difference between four-stroke Otto, Atkinson, and Miller cycle engines.  

The Otto engine as we know it today was invented in 1876 by German Nicolaus August Otto.

Although it doesn't seem that way the 1876 engine has many of the elements we see on engines today and is Otto's first true four-stroke engine. There's a crankshaft, a connecting rod, and even a camshaft. Inside we have a piston and the engine does intake, compression, combustion, and exhaust just like any modern engine. 

Of course, Otto's first engine was an instant commercial success and of course, Otto patented the design. Now this, the patent, is what brings us to Mr . James Atkinson, who like many of his contemporaries, after seeing the commercial success of the Otto engine, started developing his own engine. Now the catch is that in order to be commercially viable such an engine had to be different enough from Otto's design in order to not infringe on the patent rights.

Now Atkinson decided that the compression stroke of the Otto engine was actually something that could be improved upon, and that was to be done by reducing the length of the compression stroke in relation to the length of the combustion stroke, or the expansion stroke as some call it. In other words, the engine would spend more time making power than wasting power on compressing the air-fuel mixture. 

In 1957 US engineer Ralph Miller patented the Miller Cycle engine. Now the Miller Cycle engine relies on the same concept as the Atkinson engine and that is to reduce the power-sapping effects of the compression stroke. The big deal is that Ralph Miller chose a much simpler and much more elegant solution compared to the extremely complicated set of rods and linkages from Atkinson's original design. 

And the solution is this: keep the intake valve open longer. That's it. The construction of the engine stays absolutely the same as that of a conventional Otto engine, the only thing that differs is the valve timing.  A conventional Otto engine closes the intake valve before the compression stroke begins. This is done in order to ensure that the entire length of the cylinder is used to store and compress the air-fuel mixture leading to optimal power output. The Miller engine doesn't close the intake valve when the compression stroke begins. The intake valve is kept open during the first 20-30% of the compression stroke. An open intake valve of course means that the upward motion of the piston simply pushes some of the air fuel mixture back into the intake manifold. The piston can't compress anything until the intake valve closes.

In fact, in the late 90s, Mazda put this exact concept into practice with their KJ-ZEM engine which they installed into the Mazda Millenia / Xedos 9 /Eunos 800. The KJ-ZEM was a supercharger 2.3 V6 running the Miller Cycle.

Right after the Mazda Millenia was discontinued Toyota revived the concept behind the Atkinson/Miller cycle. Toyota's foray into this field started in 1997 with the very first generation of the Toyota Prius and its 1NZ-FXE engine. But this time instead of a supercharger we have an electric motor which is used to make up for the lack of torque and responsiveness. As we know electric motors produce instant torque and they don't sap the power of the engine like superchargers do which means that hybrid drivetrains and the Atkinson cycle are a match made in heaven which was put into practice in all of Toyota's hybrid vehicles. Now another advantage that modern technology has brought is variable valve timing or VVT. This makes it possible to run the engine in the Atkinson cycle only when this is desirable, which reduced load conditions such as highway cruising. 

Now Toyota's usage of the Atkinson cycle is perhaps the most popular to date, but Mazda definitely hasn't given up and they employed the Miller cycle yet again in a fashion similar to what we have seen in the Mazda Millenia. This time it bears the name Skyactiv-X and in addition to a small roots supercharger and the Miller cycle the Skyactiv X is also the first ever commercial engine to have Spark Controlled Compression Ignition or SCCI. 

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