P.5
Some of the thumbnails on the left are live and lead to
more images or information. |
|
POWER! - What is
it?
|
|
 |
|
Horsepower.... What exactly is it? Wherever people gather to
talk motoring, motorcycling or motor sport of any kind it is only a matter of
time before the word 'power' comes into the conversation. But very few people
really understand what horsepower is. The dictionary definition is.... "A unit
to measure the rate of doing work..." Not a lot of help for someone trying to
make crucial decisions on how to tune an engine! Yet every rider and driver
knows the feel of power... |
|
 |
|
The surge of acceleration as the throttle is opened, the clutch
dropped, and suddenly the wheels are spinning that's the feel of high
power. Even as you wind back the twist grip of your 50cc moped, it's power that
gets you moving, even though not quite so sexy. In fact, nothing moves without
power clockwork toys, elastic band propelled pellets, human beings on
bicycles, Harrier jump jets... all need power, large or small, to move at
all.
A simple
falling stone uses motive power from the kinetic energy imparted when someone
or something lifts it against the pull of gravity. So power can also be defined
as the result of converting work into movement. And the internal combustion
engine happens to be one of the most convenient ways of producing movement.
|
|
 |
|
All you need to do is pour the fuel in at one end, pull the
right knobs and levers... and instant power comes out the other end. The engine
thus converts fuel into movement. But what exactly, is fuel? Fuel is neatly
packaged heat... Anything that burns when a match is applied is potentially
capable of producing power, in this case petrol/gasoline or methanol. So the
engine is a heat converter and of course this is why, when this subject is
studied in college, it is entitled 'heat engines'. |
|
 |
|
Two stroke or four stroke, one cylinder or
sixteen, the conversion process is always carried out in the same way by
drawing in a mixture of fuel and air, compressing it, igniting it and using the
resultant combustion to drive a piston or turbine down or round, and rotate a
crank. When the engine converts fuel into power, the process is rather
inefficient and only about a quarter of the potential energy in the fuel is
released as power at the flywheel. The rest is wasted as heat going down the
exhaust and into the air or water. This ratio of actual to potential power is
called the "THERMAL EFFICIENCY", of the engine. |
|
P.6
Remember to check the thumbnail images |
|
The machine we use to measure engine performance is a
dynamometer and the way in which it works is closely tied to the explanation of
power. MEASURING POWER The term "Horsepower, was evolved during the 19th
century to describe the capability of engines to carry out a measure of work
related to the conversion of energy into motion by the horse. At that time it
was a realistic way of comparing mechanical power to "horse power". The unit of
engine horsepower, which used to be called B.H.P, the abbreviation of Brake
Horse Power, is now being replaced with the "Watt", hitherto used to quantify
the power of electric motors and other appliances. |
|
| |
|
One B.H.P is the equivalent of 746 Watts.
Both of them are a measurement
that describes the power that is actually measured at the
flywheel.
A
dynamometer is not actually capable of showing power as a direct reading, but
measures torque and R.P.M, from which power is calculated. Engine Data
Acquisition Systems (Fig. XX), are electronic measuring system that captures
information from an engine or chassis dynamometer, then automatically
calculates the power.
Torque is the amount of work that an engine is actually doing
at any given moment, that is, the turning force being exerted at the
crankshaft, and is measured in "Foot Pounds, or "Newton Metres", depending on
whether you are using the British Imperial or the International Metric System.
|
|
 |
|
Imagine an engine sited at the top of a deep
well turning a drum, which is four feet in diameter, i.e. 2 feet radius. A rope
attached to the drum is hanging down the well with a weight of 100 lbs. on the
end. As the engine turns the drum it will lift the weight. The drum is four
foot in diameter and the rope is being pulled in at two foot from the centre of
rotation; therefore the work being done or torque is measured as 2ft x 100lbs =
200 foot pounds.
The speed at which the drum is rotating is measured as
Revolutions Per Minute (R.P.M).
B.H.P is calculated as follows : |
|
| |
|
TORQUE X R.P.M
----------------------------- = B.H.P
CONSTANT |
|
|
|
The constant depends on the units of torque, which are being
measured. As we are using ft.lbs, it will be 5250, so if we say that the engine
is turning at 1000 R.P.M then: |
|
|
|
200 X 1000
----------------------------- = 38
5250 |
|
 |
|
Because we cannot calculate B.H.P without knowing the R.P.M, it
means that B.H.P is a measure of the speed at which work is done as previously
mentioned, a unit to measure the rate of doing work. To
better understand the way in which the dynamometer works, imagine anchoring a
spring balance to the ground, with a rope attached to the top eye and wrapped
around a drum with a slipknot is tightened as the drum is rotating, the rope
will be tensioned and the balance will extend to indicate this tension as a
'weight'. As the knot is further tightened, friction between rope and drum will
slow the drum and its driving engine until, at 1000 R.P.M, the spring balance
reads 100 lbs. The weight being lifted is 100 lbs and the speed of the drum or
engine will then be used in the formula to calculate the
horsepower.
If
the speed of the engine/drum were 1000 R.P.M the B.H.P being exerted would be
38.
If the
speed were 1500 R.P.M this would mean the engine was lifting the weight faster
and exerting more power to do so.
The calculation would then be: |
|
|
|
200 X 1500
----------------------------- = 57
5250 |
|
P.7
Remember to check the images for live
links |
|
The Dynamometer |
|
 |
|
So a piece of rope, a spring balance, a rev counter and an
engine fitted with a flywheel or drum to take the rope is all you need to make
a dynamometer…
Well, yes… but..!
If the throttle is wide open and nothing is moving
where is all the power going?
The answer is, that it's turning back into heat
again.
Where?
You've guessed it! Between the drum and the rope - as friction.
So although the idea of a cheap dyno sounds good, in fact the power being used,
turning into friction heat, would set fire to the whole lot.
Unless, of
course, we cool it by pouring water over it... and that's just what the modern
dynamometer does.
It uses a device similar to the torque - converter of an
automatic transmission to do the job of the rope and drum, running in a
continuous flow of cooled water to absorb the heat.
The engine turns the inner
part of the torque - converter and the water drag thus created tries to turn
the outer casing, which is coupled to a big accurate weighing machine reading
torque. |
|
1 Litre Imp consistently lapped faster than 2 Litre
Escort
 |
|
Now that we understand the meaning of power and how to measure
it just how important is it?
Does an increase in power automatically mean higher
speeds?
Not
necessarily... power is only of value when applied with suitable engineering
skill.
Smooth,
controllable power will often return better results than ultimate B.H.P peaking
over a narrow rev band.
This is often the secret behind the out-performance of many
smaller racing machines over their more powerful
contemporaries.
The examples here are from historical archives of low powered
racing machines that consistently out-performed their big brothers, regardless
of the circuit length or geometry, or vehicle handling
characteristics.
The noticeable advantages were the ability to power out of
corners more progressively.
|
|
250cc Yamaha
consistently
out-performed
1000cc Honda
 |
|
What controls power..?
How can it be
improved..? |
|
| |
|
The functions of an engine can be closely compared to those of
a human being.
It takes in air and fuel(food). It burns the fuel
(digestion).
It converts the energy released into power
(muscles).
It
discharges exhaust (bowels).
It needs an oil pump and circulation of lubrication
(heart).
It
needs a cooling system (pores).
Pistons and cylinders represent the
lungs.
The
camshaft is the brain, coordinating the whole sequence of these
functions.
The
efficiency of these individual functions effects engine performance in the same
way they would affect a human.
Poor or contaminated fuel will have low energy
content.
Bad
ignition or combustion chamber shapes will reduce the ability to digest the
fuel fully, resulting in an unpleasant and dirty exhaust. Clogged or inadequate
oil filters or a worn oil pump will result in component
failure.
A
dirty cooling system with clogged radiator pores will result in
overheating.
A
poorly designed camshaft will result in erratic breathing, adversely affecting
all other functions.
They all work together to produce the final flywheel muscle
power, however good or bad.
To understand the process we'll start with the breathing
cycle.
If we
think of the engine as an air pump then theoretically it should draw in and
exhaust its own volume of air each time it cycles - that is, once every
revolution if it's a two stroke and once every two revolutions if its a four
stroke.
In
fact, ordinary production engines don't achieve this and only manage to shift
about 80% of their volume.
This ratio of possible air pumped to actual air pumped is
called Volumetric Efficiency and this is what we have to improve to get more
power. |
|
 |
|
These two pictures of a Honda 750 four, the street version and
the full race version of the same model, typify the noticeable external
differences in the engine preparations to increase the VOLUMETRIC
EFFICIENCY.
Volumetric Efficiency = Breathing Ability =
Power
A chain of components control the breathing cycle, each one of
which depends on the others to work at it's best. |
|
| |
|
The process starts right back at the air cleaner which varies
from being a large box containing a large paper filtration and silencing
element, necessary for silent operation and engine protection under a variety
of dusty and sandy conditions, through to the light and minimal filters of
rally cars and speedway bikes, to the completely open bell mouths of full
circuit drag machines.
The
next link in the chain is carburetion.
The process of mixing the fuel and air and feeding
them to the engine in balanced doses, that is about fifteen times as much air
as fuel.
Fifteen to one - air fuel/ratio - another important controller
in a final power output! Although we call it 'carburetion', here it can also
include fuel- injection, just another method of delivering fuel to the
engine.
So,
via inlet manifolds or stubs we move to the next link and here it is where the
two stroke and four stroke engines divide. |
|
P.9
Remember to check the images for live
links |
|
The Power Train |
|
| |
|
The four-stroke train of power |
|
 |
|
The mixture enters the cylinder head and is induced through the
inlet valve into the combustion chamber.
The way in which it enters the chamber is controlled
by the port shape and finish and inlet valve timing, which in turn is
controlled by the camshaft.
The camshaft is probably the most important single component in
the four stroke engine as far as power production is concerned and it is
certainly the most complicated to design and produce.
Compression
takes place followed by ignition and combustion, the point at which four- and
two- stroke design re-unites in a common process.
Exhaust in a four-stroke
engine is again first controlled by the camshaft operating the exhaust valve,
and then by the design of the port and the exhaust system, which in turn, has a
considerable effect on exhaust efficiency. |
|
| |
|
The two-stroke train of power |
|
 |
|
There are several ways in which the two-stroke engine will work
but we will consider the modern loop-scavenge design, which is the most
commonly used production version.
As the piston rises, a depression is created in the
crankcase and the mixture is drawn in at the point where the piston skirt
starts to uncover the inlet point.
As the piston comes down, the inlet port is closed
and the charge is compressed and driven up the transfer point into the
combustion chamber). Because the mixture is being forced into the chamber under
pressure, this is really a form of supercharging and is one of the reasons that
this type of engine can produce so much power relative to its
size.
As the piston rises again, compression takes place, followed by
ignition and combustion, driving the piston down which opens the exhaust port
and drives a new charge into the combustion chamber at the same
time.
Because
these two happen together, the design of the transfer port and the exhaust
system must be just right in order to clear the foul gases and ensure a full
charge of new mixture without wasting any down the exhaust port. Although the
two designs appear to be very different, the overall function of both is the
same - fuel into power, the level of which is governed by volumetric
efficiency. |
|
| |
|
End of Chapter 1. |
|
| |
|
Chapters 2-6 study all these components and processes in
detail, referring to specific projects from which the experiences were
gained. |
|
| |
|
|
|
| |
|
|
|
