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corners. Actual propagation speeds are 20..30 m/s, proportional to the mean turbulence level (causing flame stretching); there is a fortunate correlation between this flame speed and engine speed, since turbulence is generated in the intake process, and increases with flow rate, so that the combustion process extends for nearly the same crank angle, around 30o, for all engine regimes. This turbulent flame speed is well above the laminar flame speed, which is below 0.5 m/s for all typical fuel mixtures (notice that just a 5 cm run in a car engine cylinder, would last 0.1 s at 0.5 m/s, but at 3000 rpm its crank shaft has gone 50·0.1=5 turns!). Typical energy efficiencies of these engines are low, 25% to 35% when running at nominal power, and much lower at partial load (the main air flow is strangulated), but the engine is light, very powerful and responsive (accelerates very quickly), and less expensive than diesels. Pollutant emission may be high because premixed fuel cannot burn close to the walls (the effect of walls is smaller in larger diesel engines, and most of the nearby gas is just non-premixed air), making the exhaust catalyser an environmental need. Example 1. Comparison among different power cycles Example 2. An Otto cycle engine Diesel cycle The Diesel cycle is a first approximation to model the operation of a compression-ignition engine, first built by Rudolf Diesel in 1893, and used in nearly all boats (the first in 1903), nearly all trucks (the first in 1923), many locomotives (the first one in the 1940s, but taken over by electric drive after a few decades of prominence), most cars nowadays (the first one in 1936, but it took decades to gain market), many medium-large electric auxiliary-power and cogeneration systems, and even some small airplanes. It is the reference engine from 50 kW to 50 MW, due to the fuel used (cheaper and safer than gasoline) and the higher efficiency. This is a reciprocating internal combustion engine with the same sketch as in Fig. 17.2, but substituting the spark-plug by the fuel injector (the fuel is injected at very high pressures, up to 200 MPa, to ensure immediate vaporisation). One of its key advantages compared to Otto engines is the great load increase per cylinder associated to the higher pressures allowed (the mixture of fuel and air would detonate in Otto engines at high compressions), and the further load increase associated to charging previously-compressed air (turbocharging), nowadays implemented in practically all diesel engines. Another advantage is the much better performance at part loads, since it is achieved by injecting less fuel instead of by throttling, and the torque (power divided per angular speed, M=P/), changes less with angular speed. In the ideal air-standard Diesel cycle, the working fluid is just air, which is assumed to follow four processes (Fig. 17.4): isentropic compression, constant-pressure heat input from the hot source, isentropic expansion, and constant-volume heat rejection to the environment. Fig. 17.4. The ideal Diesel cycle. Practical p-V diagrams are as in Fig. 17.3. Similarly to the Otto cycle, the main parameters of ideal and real Diesel cycles are also the size, measured by the displacement volume (that may reach more than 1 m3 per cylinder in large marine engines), the compression ratio, r=V1/V2 (with a typical range of 16..22, limited just by strength), thePDF Image | THERMAL MACHINES AND HEAT ENGINES
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