What’s ineffective crank angle
the angular position around TDC/BDC where, for arelatively large angular movement of the crankshaft, there is a small linear movementof the piston.
What’s valve lead
Opening the Inlet
Valve early (20º~30º)
ensures that the valve
is FULLY OPEN by TDC
What’s valve lag
Keeping the Inlet
Valve to remain
open after BDC
(50º~70º) allows
the momentum of
the incoming
mixture to increase
the mass of the
induced charge
What’s advanced ignition
It takes a finite time to ignite the mixture and for the flame front to
expand across the
crown of the piston
• The spark is initiated
early (before TDC,
~30º), during the
compression stroke, to
achieve maximum
pressure at ~10 degrees after
TDC
How is max pressure and temperature achieved after tdc
IgnitionDue to advanced ignition, the combustion is completed
after TDC, ideally past the ineffective crank angle.
This means, maximum pressure and temperature are
achieved after TDc
How does the exhaust valve prevent back pressure in the power cycle
By the time the piston has passed 90 degrees on the
power stroke most of
the pressure energy
has been expended
• The Exhaust Valve
OPENS EARLY (40º~60º)
to prevent back
pressure which resist
upward movement of
the piston
How is more space allowed
for the incoming
air/fuel mixture in the exhaust phase
The Exhaust Valve
remains open after
TDC (~30º)
• The momentum of the
moving exhaust gas
removes the last of the
burned gases
What is valve overlap and how does it benefit the engine
period when
both inlet and exhaust
valves are partially open
The exhaust gases
flowing out help the
air/fuel mixture to flow
in
• This improves
Volumetric Efficiency.
The mass of the induced
charge is increase
Formula for brake horse power
BHP= (force x length of arm x 2pi x RPM)/ 33000
( BHP = Torque x RPM)
What’s indicated horse power
the maximum theoretical that the engine could deliver
How’s indicated horse power calculated
IHP = PLANK
P- indicated mean effective pressure
L- length of the stroke
A- area of piston crown
N- number of power strokes per minute
K- number of cylinders
What’s friction horse power
the power lost due to friction between engine elements and due to the accessories installed on the engine
How can friction horse power be calculated from brake horse power and indicated horse power
FHP = IHP - BHP
What’s specific fuel consumption
Specific Fuel Consumption represents how
efficiently fuel is used per unit time in order toproduce a given amount of power.
• It’s expressed as MASS of fuel burnt per unit of TIME for each unit of POWER (BHP) produced.
– A reduction in power for the same (or greater) weight of fuel burnt is an increase in SFC
– A reduction in fuel burnt for the same (or more) power is a decrease in SFC
• SFC depends both on engine design and on how the engine is handled by the pilot
How does air density effect power output
The mass of air consumed by an engine determines the power
Density is affected by pressure, temperature and humidity
How does manifold absolute pressure effect power outpud
Pressure in the induction system between throttle valve and
intake valve. The higher MAP, the higher the power output.
How does exhaust back pressure effect power output
As ambient pressure reduces with altitude the pressure acting on
an open exhaust pipe also reduces. This improves scavenging of
the exhaust gas.
How does mixture ratio effect power output
Different combinations of mass of air and mass of fuel produce
different combustions, and hence different power.
– Needs to be corrected when density varies (e.g. climb)
