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cut-off ratio, rc, or the mean effective pressure (in the range 1..2 MPa), or the maximum pressure (in the range of 3 MPa to 20 MPa), and the speed (with a typical range of 100..6000 rpm). The energy efficiency can be expressed as: c 1 1 r 1 (17.3) e,Diesel r1 (r 1) c Real compression-ignition engines take ambient air (often after a first stage compression) and compress it (inside the cylinder) so much, rising the temperature accordingly, that the fuel burns as it is injected (after a small initial delay due to vaporisation and combustion kinetics). The external air compression is performed in a centrifugal compressor driven by a centrifugal turbine moved by the exhaust gasses (turbocharger); the air is cooled after external compression (inter-cooler) before further compression within the cylinders, to increase the efficiency, as explained in Multistage compression. The higher pressures in Diesel than in Otto engines require a robust engine-frame and delicate fluid injection hydraulics (with injection pressure up to 200 MPa), but the wider range of fuels (from gas- oils to fuel-oils), their better safety (those fuels are combustible but not inflammable at ambient temperature), the better fuel control by direct injection, longer durability and better economy, are making Diesel engines to take over traditional gasoline-engines markets (for heavy-duty applications it has always been unrivalled). The fuel-injection system is the heart of a diesel engine, having to supply very precise minute amounts of liquid at a very high pressure through injection holes of some 0.1 mm in diameter and 1 mm long, where the liquid usually cavitates (it enhances spraying and burning); a great advance took place in late 20th century with development of the common-rail injection system, in which a very-high pressure reservoir (the common rail) separates the task of pumping from the task of metering (before that, the injection pump had to control pressure and fuel volume at the same time); the electronic metering has brought not only better fuel control, but a dependence on electricity for the working of diesel engines, as has always been for spark engines. Typical energy efficiencies are from 30% to 54% (based on the lower heating value of the fuel), the latter efficiency (the largest of any single thermal engine), being achieved in large two-stroke marine low-speed engines with bores larger than 0.5 m: first, because the thermal losses decrease with size, and second, because the very low speed (some 100 rpm instead of the typical 3000 rpm for a car engine) allows for a more complete combustion (more time to burn, and burning nearly without volume change) and decrease friction losses (in spite of the fact that the mean piston speed stays at some 6..7 m/s for the whole range of reciprocating engines: from the 1 cm3 50 W model, to a 1 m3 5 MW per cylinder 'three-store castle' large marine engine). Most large Diesel engines are supercharged, i.e. fed with compressed air instead of atmospheric air, usually by means of a turbocharger (a small compressor shaft-coupled to a small turbine driven at very high speeds (up to 100 000 rpm) by the exhaust gases; some 10% of the fuel energy goes through that shaft), with an intermediate cooling of the compressed air before intake to the cylinders (intercooler). The two-stroke cycle is better suited to Diesel engines, since only air is used to sweep the burnt gases (scavenging), and not fresh mixture, but, because of the difficulty in lubricating, it is only used in the largest marine engines (10 MW..100 MW), where residual fuel (must be preheated to flow) can be used. Mixed cycle The mixed (or dual, or Sabathé, or Seiliger, or 5-point) cycle, sketched in Fig. 17.5, is a refinement to both Otto and Diesel cycles, at the expense of an additional parameter, the heat-addition pressure ratio, rp=p3/p2.PDF Image | THERMAL MACHINES AND HEAT ENGINES
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