The lines of constant power show the curve of hyperbolas. Apart from that, lines of constant specific fuel consumption, the so-called shell curves, are drawn in. The specific fuel consumption corresponds to the fuel mass flow relative to the effective engine power.
It becomes clear that the fuel consumption of a motor vehicle is significantly influenced by the transmission ratio between engine and drive axle, since the same amount of power can be achieved at different speeds at a distinct specific fuel consumption. For example, by decreasing the speed from point 1 in the figure to point 2 at equal power it is possible to reduce consumption.

Diesel Engines
The diesel engine today, is exclusively manufactured on a large scale as a reciprocating engine. Recently, discussions on the development of rotary piston engines working on the diesel principle have been again re-opened. So far, they have however failed as a result of the high manufacturing expenses involved.
The Diesel engine differs from the Otto engine, in that it compresses pure air which is followed by the injection of liquid fuel under high pressure at the end of the compression stroke. Thanks to the large compression ratio (ε4= 12 to 22), the engine can also be fired at cold starts in the presence of sufficient hot air. To summarize the concept, these characteristics lead to an internal mixture formation and auto-ignition of a non-homogeneous mixture.
The operation with air-fuel-ratios that are higher compared to the Otto engine, meaning an extremely lean mixture (part load λ5 = 3 to 6), implies lower heat transfer and dissociation losses (losses due to incomplete combustion).
In addition, the diesel engine being quality-controlled by fuel injection (λ ≠ const.), the flow losses in the part-load area are low unlike in the case of the Otto engine where throttling occurs as a result of a quantity controlled strategy (λ = const.). All of this leads to an improved efficiency.
λ>1 = excess of airν "lean" mixture
λ'1 = lack of airν "rich" mixture
Diesel engines are can be classified into Direct-Injection Diesel Engines and Pre-Chamber or Turbulence-Chamber Engines. While direct-injection diesel engines have been in dominating the commercial vehicle segment due to their low consumption for a long time, the high noise and low running culture has prevented its implementation in passenger cars. Meanwhile these problems have been largely solved in new designs.
The torque characteristics dependent on the engine speed are generally flatter compared to the Otto engine (more power at low speeds). As a consequence of higher pressures inside the cylinder, the diesel engine is made of more robust components. This results in a larger connecting-rod and piston mass as well as to a reduction of the permissible engine speed compared to the Otto engine.
In order to enhance the efficiency of diesel engines, turbochargers are often employed. For an additional improvement of the efficiency, they are used in combination with an intercooler. In contrast to Otto engines, a simultaneous improvement of the consumption and emission behavior is possible.
Apart from the Diesel and Otto engine described above, there are special forms of the piston engines which use hybrid combustion methods. This term summarizes methods that have the characteristics of both Diesel as well as Otto cycles. The stratified-charge engine is one example. Fundamentally it corresponds to the Otto engine but burns a non-homogeneous, partially very lean mixture. Its application is however essentially problematic since it does not allow for the use of a conventional 3-way catalyst. Earlier designs have not seen considerable application. Direct-injection gasoline engines have recently been developed to an extent where they can be produced in series. A new catalyst is used in these engines. This engine concept is applied in order to achieve identical fuel consumption values as the diesel engine (Mitsubishi).