Page 387 - Advanced thermodynamics for engineers

376 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

operating on an Otto cycle, as many engines approach in real life, can achieve the Carnot efficiency if an
infinitesimal amount of fuel is burned, and the engine produces zero power output. This agrees with the
concept developed in Chapter 6, when endoreversible engines were examined to find their efficiency at
maximum power. This idea was taken further, and it was shown that friction can have a dramatic effect
on the brake thermal efficiency of a reciprocating engine, and the compression ratio at which it occurs.
Finally, an engine simulation was ‘exercised’ to see the effect of changing various parameters in
isolation: something that is practically impossible in the real world. This enabled the effects of
compression ratio, engine speed, air–fuel ratio, ignition timing, flame speed and egr to be investigated.
The results give an insight into the effect of these parameters on cylinder pressures and temperatures.
They show that combustion is not an instantaneous process but one that can become quite protracted
under certain circumstances, e.g. high engine speeds with slow flame speeds, or the wrong ignition
timing; and the broad effect of operating at extreme air–fuel ratios.

16.8 PROBLEMS
P16.1 (a) One of the main problems encountered in the design of a diesel engine combustion system
is the mixing of the air and fuel sufficiently rapidly to ensure complete combustion. Explain,
using diagrams, how these problems are catered for in the design of
(i) large automotive diesel engines;
(ii) the smallest automotive diesel engines.

Give two relative advantages of each type of combustion system.
(b) Compare and contrast the combustion systems of diesel and spark-ignition engines in the
forms they are applied to passenger cars.
P16.2 A method of reducing (improving) engine fuel consumption and reducing the emissions of
NO x in a spark-ignition engine is to run it lean, i.e. with a weak mixture. Discuss the problems
encountered when running engines with weak mixtures, and explain how these can be
overcome by design of the engine combustion chamber.
P16.3 Calculate the ignition delay period (in deg ca) in a diesel engine using Eqns (16.15a) and
(16.15b) using the conditions at the beginning of fuel injection that exist after compression in
the engine with the following initial parameters.
Compare the results to those calculated by the equation proposed by Watson (1979):

3:52e ð2100=T igÞ
t ig ¼ 1:022
p ig
Conditions at ivc: p ¼ 1.3 bar; T ¼ 310 K; compression index, n c ¼ 1.35. Assume CN ¼ 50.

(a) Effective compression ratio, r ¼ 10:1; engine speed, N ¼ 2000 rev/min.
(b) Effective compression ratio, r ¼ 15:1; engine speed, N ¼ 2000 and 4000 rev/min.
Comment on these results – do they seem reasonable?

[(a) 78.7; 27.8; 5.61; (b) 16.92; 10.63; 2.57; 33.84; 21.27; 4.59]