1991-02-01

Estimate of Total Engine Loss and Engine Output in Four Stroke S.I. Engines 910347

In the previous reports (SAE PAPER 880268, 900223), a relationship between dimensional specifications and total engine loss (Pmf) measured by motoring method under certain conditions was established by analyzing 300 different types of mass-produced engines of which engine speed for maximum power output (Nepsmax) was obtained between 8,000 and 16,000 rpm, and thereby an empirical formula was proposed (Pmf).
In this paper, it is proved that brake mean effective pressure (Pme) can be estimated by using the empirical formula for Pmf and modified indicated mean effective pressure (Pmi*) obtained from analyzing various types of motorcycle engines consisting of different cylinder configurations, valve sizes and numbers.
At first, the relationship between the characteristic values of engine and Nepsmax is described, and methods to obtain higher power output by increasing engine speed are also discussed.
It was found that the proposed empirical formula for Pmf is applicable to high revolution engines which run up to 20,000 rpm and that a reduction in Pmf can be achieved by utilizing an engine with short-strokes, multi-valves and multi-cylinders.
Under optimum operating conditions, the values of modified indicated mean effective pressure (Pmi*) [≡ (Pmf + Pme) / (κ(ε)ηυ)] have shown to be substantially constant and agree well with the value of modified Pmi̅ (Pmi̅*) which is obtained from the indicated pressure diagrams of 300 cycles under same optimum operating conditions.
Therefore, it is concluded that net output (Pse) can be also estimated at planning stage of a new engine layout with the value of ηv calculated numerically, the constant value of Pmi* and the value of Pmf calculated from the empirical formula.

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