Question

A conventional spark ignition engine operating with gasoline will not run smoothly with…

A conventional spark-ignition engine operating with gasoline will not run smoothly (due to incomplete combustion) with an equivalence ratio leaner than about f=0.8. Itis desirable to extend the smooth operating limit of the engine to leaner equivalence ratios so that at part throttle operation (with intake pressure less than 1atmosphere) the pumping work is reduced. Leaner than normal operation can be achieved by adding hydrogen gas (H2) to the mixture in the intake system. The addition ofH2 makes the fuel-air mixture easier to burn.
(a)
The fuel composition with “mixed” fuel operation is H2 + C8H18 — one mole of hydrogen to every mole of gasoline, which is assumed the same as isooctane. What is thestoichiometric air/fuel ratio for the “mixed” fuel?
(b)
The lower heating value of H2 is 120 MJ/kg and for isooctane is 44.4 MJ/kg. What is the heating value per kilogram of fuel mixture?
(c)
Engine operation with isooctane and the mixed (H2 + C8H18) fuel is compared in a particular engine at a part-load condition (brake mean effective pressure of 275 kPaand 1400 rev/min).
You are given the following information about the engine operation:
Fuel
C8H18
H2 + C8H18
Equivalence ratio
0.8
0.5
Gross indicated fuel conversion efficiency
0.35
0.4
Mechanical rubbing friction mep
138 kPa
138 kPa
Inlet manifold pressure
46 kPa
?
Pumping mep
55 kPa
?
Estimate approximately the inlet manifold pressure and the pumping mean effective pressure with (H2 + C8H18) fuel. Explain your METHOD and ASSUMPTIONS clearly. Notethat mechanical efficiency ?m is defined as
?m = bmep/imepg = bmep/(bmep + rfmep + pmep)

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