Page 223 - Advanced thermodynamics for engineers

210 CHAPTER 10 THERMODYNAMICS OF COMBUSTION

Light distillates Fuel to run
(chemical feedstock) refinery Heavy fuel
10% 6% or fuel oil
(marine)
17%

Petrol
25%

28%
17% Middle distillates (gas
Kerosene (paraffin, 1% oil, diesel, heating oil)
aviation fuel) Gases (butane,
propane)
FIGURE 10.1
Typical constituents of a barrel of crude oil.

sensors are fitted to engines, and these retard the ignition when knock is detected. Octane number is
evaluated by running a calibration engine on the fuel and comparing its performance with the engine
using a mixture of isooctane (100) and heptane (0). The octane number is equal to the proportion of
octane in the calibration mixture. A high octane number does not indicate that the fuel has more energy
than one with a low octane number. Two octane numbers are quoted: the Research Octane Number
(RON) determined simply by varying the compression ratio, and the Motor Octane Number (MON)
which is assessed under more realistic operating conditions. MON is usually between 8 and 10 points
lower than RON.
Fuels for compression ignition, or diesel, engines require a high cetane number (CN): this is a
measure of the ability of the fuel to ignite after compression – it is almost the inverse of the octane
number. Higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. There-
fore, higher cetane fuel usually causes an engine to run more smoothly and quietly, and reduces diesel
‘knock’, which occurs when the fuel that has been initially injected into the cylinder suddenly ignites
(see Fig. 16.7). Generally, diesel engines operate well with a CN from 40 to 55, and modern high-speed
diesel engines operate more effectively with higher CN fuels. Diesel fuel is about 11% denser than
petrol, while the lower enthalpy of combustion is about the same – hence the energy per unit volume is
about 11% higher, so (for equivalent energy of combustion) a diesel engine should have a fuel con-
sumption (miles/gallon) at least 11% higher.
In general, for any given aircraft turbine application, the optimal fuel is the one that represents the
best compromise to the various problems confronting the fuel companies, the engine and aircraft
manufacturers and the operators. For civil aircraft the main requirements are safety, reliability, low
cost and ease of handling. For military aircraft fuel cost is of secondary importance compared with