Homework 4
Chapter 9: Turbocharging
9.A An engine is being developed as follows:
2.5 ltr SI four Cylinder, 100mm bore. Rev limit will be 7,000 RPM. The normally
aspirated version is planned to make 155 Hp at 6800 RPM with a Volumetric Efficiency
of 95% and mechanical efficiency of 85%. A/F ratio will be 14.7:1 with 44MJ/kg CV.
Ratio of specific heats is standard (1.4) and pressures and temp are 25C and 1 bar
(101Kpa). Indicated (Arbitrary overall) efficiency is 60% of the otto cycle. Cv =
950MJ/kgK. R=287J/kgK
The turbocharged version will make 300 HP and run 8:1 compression ratio and is
expected to have the same efficiencies. The engine will develop no boost under 2500
RPM, rise linearly to 80% max boost at 3500 RPM and reach full boost at 4,000 RPM.
Boost will trail off from 100% to 90% from 6500 to 7000 RPM. Assume average
compressor efficiency of 70% as a first cut.
a) What compression ratio is needed for the NA engine to make this power?
Perform all calculations by hand and show all work INCLUDING:
i.
Convert HP to kW and calculate desired work per cycle
ii. Calculate mass of air and fuel per cycle
iii. Calculate the required otto efficiency to make this power
iv.
Calculate all pressures and temperatures for the ideal air standard otto
cycle, heat flows, otto efficiency.
(hint see exam question problem 3 and work backwards)
Make up an excel spreadsheet with the same calculations.
b) The goal is to make 300 HP at 6,500 RPM. Do calculations by hand and then
generate an Excel spreadsheet to allow for multiple RPM and boost calculations.
First assume 1.0 bar boost and 6500 RPM
i.
ii.
iii.
iv.
v.
vi.
Calculate T1 & T2s
Calculate T2
Calculate the density ratio
Calculate air flow per minute (CFM, LB/Min and kg/sec)
Calculate fuel flow per minute (kg/sec)
Calculate power in kW and Horsepower.
c) Add a column to your spreadsheet and increase/decrease boost until 300HP is
attained. Report i..vi above
d) Calculate power and torque for BOTH NA and Turbocharged and graph the
results (RPM as the X axis). On a separate axis graph boost.
e) Comment on the need for a waste gate.
f) Select an appropriate turbocharger from the Turbonetics page, print out the
compressor map, and graph results.
9.B Perform the same calculations for a 3.6 liter 6 cylinder. Assume it will make
proportionately the same amount of power and answer as above (simply copy the
spreadsheet to an additional sheet, then change parameters)
Select turbocharger s for a single and twin-turbo installation.
9.C Add intercoolers to both of the engines above. Assume a 1psi pressure drop and 80%
efficiency. Calculate density ratio and resulting power. What happens at low RPM (zero
boost)? What effect will this have on Lag? Where should an intercooler be placed in the
944? How about in the 911 (rear engine)?