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Why Do Rotary Engines Offer Bad Mileage?

The rotary engine has a constant crankshaft, and the corresponding cylinder block revolves around it. The engine has its plus points and some drawbacks, but the low mileage offered by the rotary engine is the matter of concern.

1. The Inefficient Engines

The thing is; the rotary engines end up producing excessive power. The engines, in turn, are not built to handle the excessive power. As a matter of fact, the more power the more fuel is burnt as the revs of the vehicle get higher. This is the simplest reasons that explain why rotary engines offer a bad mileage.

2. Seal Leakages

The temperatures of each chamber of the engine house differ. This tends to be problematic when the diverse expansion coefficients of the materials lead to faulty sealing. Furthermore, a case of seal leakages occurs that results in leakages of combustion gas into the other chambers. Such wastage of gas directly means low fuel economy.

3. Low Compression Ratio

Compression ratio is the overall ratio of the maximum and the smallest volume of the cylinder at any given point within the internal combustion engine. The best compression ratio that has been recorded on a rotary engine is 11:1, which is not okay in terms of what modern engines can offer. The best compression ratio for a petrol engine is 10:1.

4. Long Combustion Chamber

The combustion chamber of a rotary engine is designed to be very long. This may work well to some extent, but the high surface area to volume ratio makes things tricky. You may ask, ”How may it affect the fuel consumption,” to which the answer lies in the extended cooling time of the fluids. If the cooling is suffered, so is the overall fuel economy.

5. The Case Of Fixed Ports

The valves or camshafts are missing in a rotary engine. This problem with the rotary engines leads to the inability of meddling with the valve timings, i.e., the case of “no valve timing.” The port time can only be changed by machining the ports of altering the piston skirt. So, when the valves do not open and close at a synchronized timing, the engine performance deteriorates. In simple words, the rotary engine fuel economy turns out to be bad.

The most common mistake made by rotary enthusiasts intent on supercharging their engines is to supercharge a stock, unmodified non-turbo engine. 

Unless you are content to use the power gain only occasionally, and even then only briefly, you run the very serious risk of catastrophic engine failure. Sustained use generally brings failure, and the more common failures include broken apex seals and flattened apex seal springs. On occasion a stationary gear breaks, or a rotor gear moves away from the rotor and jams against a side housing, or a bearing fails due to overheating. With any of these failures, a complete engine rebuild is required.

The causes of these problems, and others, are many. Superchargers generate heat loads well in excess of what a stock engine can handle: the stock water and oil cooling systems are overwhelmed and simply cannot carry away the excess heat fast enough.

Additionally, the compression ratio commonly found in non-turbo engines is not low enough for supercharger applications. Depending on horsepower requirements, a compression ratio as low as 7.5:1 may be in order for reliable operation. The higher the boost level you desire to run, the greater the likelihood you will need to address the issue of a lowered compression ratio. In our experience, we have found that 5 psi., approximately, is the threshold above which the stock, non-turbo compression ratio is no longer appropriate.

As the above comments would suggest, we do not recommend supercharging an otherwise stock, “non-turbo-based” unmodified engine. When you weigh the anticipated power gains against the very real likelihood of a premature, and costly, engine failure it’s likely not worth the headaches.

If you are willing to build an engine that is capable of handling the increased heat loads that superchargers develop, the following tips will prove beneficial, increasing the likelihood of a long-life engine.

How It Works

A rotary engine is a barrel-shaped internal combustion engine that lacks many of the major parts you’d find in a conventional piston engine. For one thing, there are no pistons chugging up and down. Rather, rounded triangular rotors—most often two, but sometimes one or three—spin around a shaft through the hollow barrel.

Fuel and air are pumped into the spaces between the rotors’ sides and interior walls of the barrel, where they ignite. The rapid expansion of exploding gases turns the rotors, thus generating power. The rotors fulfill the same task as pistons in a piston engine, but with far fewer moving parts, making a rotary engine lighter and smaller than a piston engine of equivalent displacement.

The basic design is a century-old one. Felix Wankel himself was a German engineer who came up with his version of a rotary engine in the 1920s. Being busy with warmongering on behalf of the Nazi party, however, he didn’t get the chance to develop his vision too far until 1951, when German automaker NSU invited him to design a prototype.

Wankel Engines

Try not to get caught up in the semantics of what to call this engine. Commonly referred to as a rotary engine (even by Mazda, though often this refers to a rotating piston-cylinder-based layout), the Wankel engine was last used in production in the Mazda RX-8. There are no pistons, camshafts, or connecting rods.


  • Simplicity: rotary engines can have as few as three main moving parts, versus more than 40+ for piston-cylinder based engines. Fewer moving parts typically leads to better reliability.
  • No reciprocating mass: this allows rotary engines to rev high, and also run very smoothly.
  • Weight: rotary engines are compact and offer great power-to-weight ratios.
  • Power delivery: because of the way a rotor rotates, power delivery lasts for more of the rotation of the crankshaft versus a piston-cylinder engine, resulting in super smooth power delivery.
  • Size: rotary engines are compact, allowing for easy packaging.


  • Fuel economy: the exhaust often includes unburned fuel, on top of which Wankel engines typically have low compression ratios, resulting in poor fuel efficiency.
  • Emissions: unburned hydrocarbons leaving the exhaust makes it difficult to pass emissions regulations.
  • Rotor sealing: due to the varying temperatures throughout the combustion chamber, the apex seals expand and contract making it difficult to create a good seal, leading to inefficient power production.
  • Oil burning: by design, Mazda Wankel engines burn oil to help maintain the longevity of the apex seals. Not only does this further increase exhaust emissions, but it requires the owner to add oil periodically.

Mechanical Operation

A rotary engine uses a triangular-shaped rotor to divide the space inside the engine, enabling a standard four-stroke cycle of intake, compression, ignition and exhaust. The moving rotor transports fuel to the various engine compartments for each leg of the cycle. In this way, it resembles a reciprocating piston engine. Rotary engines can be built with any number of rotors, much like the multiple number of cylinders offered in piston engines. The rotors engage a drive shaft, which then powers the vehicle’s drive mechanism (the propeller of a plane, or wheels of a car).


One of the major advantages of a rotary engine is its mechanical simplicity. A rotary engine contains far fewer parts than a comparable piston engine. This may decrease the cost of design and manufacture. This also leads to decreased weight. Compared to standard reciprocating piston engines, rotary engines contain no valves, camshaft, rocker arms, timing belts or flywheel. All this means decreased weight, fewer opportunities for malfunction and easier repair. When rotary engines were first developed, they were used to power aircraft, taking advantage of the rotary engine’s high power-to-weight ratio.