Design and Simulation of Two Stroke Engines

where:
Wj = imep x Vsv x (work cycles per second)
Chapter 1 - Introduction to the Two-Stroke Engine
= imep x Vsv x rps - for a two - stroke engine
rps d-5-25)
= imep x Vsv x - E — - for a four - stroke engine
For a four-stroke cycle engine, which has a working cycle lasting two crankshaft revolutions,
the working cycle rate is 50% of the rps value, and this should be inserted into Eq.
1.5.25 rather than rps. In other words, a four-stroke cycle engine of equal power output and
equal swept volume has an imep value which is double that of the two-stroke engine. Such is
the actual, if somewhat illogical, convention used in everyday engineering practice.
The indicated torque, Zj, is the turning moment on the crankshaft and is related to power
output by the following equation:
Wj = 27iZjrps (1.5.26)
Should the engine actually consume fuel of calorific value Cfi at the measured (or at a
theoretically calculated) mass flow rate of rhf, then the indicated thermal efficiency, r\\, of the
engine can be predicted from an extension of Eq. 1.5.23:
power output Wj CI 5 271
rate of heat input mfCfl
Of great interest and in common usage in engineering practice is the concept of specific
fuel consumption, the fuel consumption rate per unit power output. Hence, to continue the
discussion on indicated values, indicated specific fuel consumption, isfc, is given by:
fuel consumption rate rhf fl 5 281
power output Wj
It will be observed from a comparison of Eqs. 1.5.27 and 1.5.28 that thermal efficiency
and specific fuel consumption are reciprocally related to each other, without the employment
of the calorific value of the fuel. As most petroleum-based fuels have virtually identical values
of calorific value, then the use of specific fuel consumption as a comparator from one
engine to another, rather than thermal efficiency, is quite logical and is more immediately
useful to the designer and the developer.
1.6 Laboratory testing of two-stroke engines
1.6.1 Laboratory testing for power, torque, mean effective pressure and specific fuel consumption
Most of the testing of engines for their performance characteristics takes place under
laboratory conditions. The engine is connected to a power-absorbing device, called a dyna-
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