Engine Performance

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Engine Terminology
Measuring Engine
Performance
Lesson 7
March 2008
Definition of Engine Torque
Turning or twisting force that causes a body to turn (rotate).
Torque = Length x Force Applied
Engine torque is a rating of the turning force
(torque) at the crankshaft end. Engine torque
specifications can be found in shop manuals.
Engine Torque Specs
Given
in a shop manual
Torque
450
specification example:
ft lb @ 3000 rpm
engine
would be capable of producing a maximum of
450 ft lb of torque at an operating speed of 3000
revolutions per minute
The
(N•m)
metric unit for engine torque is newton-meters
Definition of Horsepower
Horsepower is the rate at which torque is
produced.
One horsepower is equal to 33,000 footpounds of work in one minute.
One horsepower is equal to 746 watts, 545
Btu, 1055 joules, or 252 gram-calories.
1 HP is equal to 33,000 ft.lbs. of Work
Horsepower Formula
Remember laws of mechanics (physics)
work = Distance (ft.) Moved x Force (lbs.) Applied
Horse power example
BHP & IHP
BHP – Brake Horsepower
usable power at the engine crankshaft
IHP – Indicated Horsepower
power formed in the combustion chamber
There is a third HP rating that is important
**WARNING**
Horse Power Loss
Friction and Load (HP robbers)
Can you describe the difference between friction and
load?
Engine Friction
What are the components that cause engine friction?
Engine Load
What components cause engine load?
Does it not take horse power to drive these accessories?
FHP (Frictional Horse Power)
The power needed to overcome engine friction and load
IHP (Indicated Horse Power)
_
FHP (Frictional Horse Power)
=
BHP (Brake Horse Power)
Efficiency
How efficient is the four stroke cycle engine?
5% loss to engine friction and load
2 % chassis and transaxle weight
3 % tire resistance
2 % loss to aerodynamics
35 % Cooling system
35 % Exhaust system
20 % or less to do work
.80 cents of every dollar thrown away
Designed Loss of Efficiency
Do you remember our discussion about engine
friction and load?
Now you can see the impact they have on
efficiency.
2 Greatest Wastes of Power?
Exhaust System 35%
Cooling System 35%
Thermal Efficiency
Thermal efficiency is the
measured comparison between
horsepower output in relation to
the amount of fuel burned to
create this power.
To look at it differently, thermal
efficiency is the measurement of
how well the engine uses the
energy (heat) produced from
combustion (burning fuel charge)
to turn the crankshaft.
Normal rating is 20-30 %.
Keep the Heat
In general, less heat
rejected to the cooling
system or atmosphere
results in more heat energy
to be converted to
mechanical energy

Exhaust coatings and
wraps

Coated piston crowns

FHP / NHP /GHP /THP
NHP – Net Horsepower – maximum power developed by an
engine when all accessories on
GHP – Gross Horsepower – maximum power developed with
only minimum accessories on
THP – Taxable Horsepower – rating of engine size
Formula
thp = bore squared x number of cylinders x .04
Reducing Friction

Low tension piston rings

Thinner piston rings

Low tension valve springs
Lighter valves allow lower tension
valve springs


Low-friction coatings

Main and rod bearings

Piston skirts

Cam lobes and followers
Mechanical Efficiency
Mechanical efficiency compares the difference between Brake
Horsepower (BHP) and Indicated Horsepower (IHP).
If 70-80% mechanical efficiency is normal, then the lost
horsepower was caused by our horsepower thieves and
Frictional Horsepower.
FHP(frictional) = IHP(indicated) – BHP(brake)
Example :
30 (IHP) – 25(BHP) = 5
FHP
Engine Efficiency
Engine efficiency is the ratio of BHP (Brake Horsepower)
to the power produced by the burning of a fuel charge
(combustion) in the engine cylinders.
BHP in relation to Fuel quantity = Engine efficiency
As previously seen, only an average of 20% heat energy is
available to do work (turning crankshaft).
Engine Dynometer
• measuring
engine performance without driveline
frictional losses
Chassis Dynometer

Measures the horsepower delivered to the rear wheels

Indicates the amount of horsepower available to propel the car
 Accounts
for any power consumed by the drive train
Dyno Performance Chart
Example of Engine Rating
2.0
liter
150 @
6,400 rpm
Engine HP (Dodge.com)
141 @
5,200 rpm
135 @
5,200 rpm
Lb-Ft
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