We've seen in the previous article how torque and power are
defined and calculated - now let's look more closely at how they relate to
engine design. The concept of an engine's torque output seems to be confusing
to many people judging by newsgroup threads but it needs to be clearly
understood if one is to design the best ways to improve power output.
Torque can be thought of as the instantaneous turning force
generated at the crankshaft. As such it is a measure of the amount of energy
being developed in the engine during EACH operating cycle - in other words a
function of the amount of air/fuel mixture being burned per cycle.
Power can be thought of as a measure of the amount of energy being
developed in the engine per minute - in other words a function of the amount of
air/fuel mixture being burned per cycle multiplied by the number of cycles per
minute. So power is torque times speed as we have already seen.
To increase torque we need to either process more air/fuel mixture
per cycle or extract more energy from the air/fuel that is processed. We can do
the latter in a variety of ways including:
1) Improving mechanical efficiency with attention to design of such things as
bearings, piston rings etc.
2) Increasing compression ratio which extracts more energy from the mixture
being burned.
3) Optimising fueling and ignition timing.
We'll look at the above another time - for now lets concentrate on getting more
air/fuel mixture into the engine.
We can simplify even further by leaving out the fuel part of
"air/fuel" mixture as this is really a calibration issue and falls
under 3) above. It is increasing the air consumption that is the real problem
and in fact it is not a bad idea to think of an engine as an air pump. The
better we can make this pump work the more torque and power we can generate.
Our problem of increasing torque output has now ended up as a problem of
getting more air into the engine each cycle. There are only 2 ways to do this:
1) To increase the engine size. This is not always an option or at
least not always a cost effective option. We may be running in a racing class
where the engine size is limited or we may own an engine where parts such as
longer stroke crankshafts or bigger pistons are expensive. As a general rule
though, a bigger cylinder will process more air per cycle than a smaller one
unless limited by other factors.
2) To increase the filling efficiency of the cylinders - i.e. to
increase "Volumetric Efficiency". If a cylinder is 500cc in volume
but processes only 400cc of air each cycle we can say that the volumetric
efficiency is 80%. In fact to be absolutely correct it is normal to express VE
in terms of mass of air not volume but that is getting more complicated than is
needed for now. To get into the cylinder, the air has to pass through the carb
or injection system, the inlet manifold and finally through the port and valve.
The more restrictive to flow each of these components is, the harder it is for
the air to get through them. By testing each of these items on a flow bench and
modifying them to increase their flow capacity we can allow the air an easier
passage into the cylinder and this will increase not only VE and therefore
torque but also allow the engine to run at higher speeds and increase peak
horsepower.
In fact the ultimate horsepower potential of any engine is really
a function of the flow capacity of the induction system. By just increasing
engine size, say with a longer stroke crank, we will increase torque
at low rpm but not necessarily increase peak horsepower by much at all. The
flow capacity of the induction system imposes the ultimate limit on the amount
of air that the engine can process per minute and whether we have a small
engine running at high speed or a big engine running at low speed, it is total
airflow per minute that matters. The only real difference between a 3 litre car
engine producing 200 bhp and a 3 litre Formula 1 engine producing 800 bhp is
the flow capacity of the cylinder head.
We can also increase airflow per cycle by opening the valves for
longer or to a higher lift. This has its downside though because long duration
camshafts don't work well at low engine speeds and while this might be ok for a
race engine it is not what we want for a road engine. Increasing the airflow
capacity of the induction system has very little downside although there can
still be minor adverse effects on low speed performance. As a general rule it
is much better to have a high flow induction system and be able to use a short
duration camshaft to achieve the desired horsepower than vice versa. The most
restrictive part of the induction system and therefore the part that often
shows the greatest benefits from being improved is the cylinder head. In fact
the flow efficiency of the cylinder head is the key to good engine design and
is the reason why modern engines are increasingly being designed with 4 or more
valves per cylinder rather than 2. More valves mean more valve area and it is
valve area that limits flow. Cylinder head design merits its own section and
we'll discuss it in detail in other articles.
To conclude our look at torque and power let's see what sort of
figures engines actually produce. The charts below show the manufacturers
quoted outputs for a variety of road engines.
|
2 VALVE PER CYLINDER ENGINES |
|||||
|
ENGINE |
CAPACITY |
PEAK |
PEAK |
POWER |
TORQUE |
|
FORD CVH |
1597 |
96 |
98 |
60 |
61 |
|
GOLF GTi |
1781 |
112 |
117 |
63 |
66 |
|
PEUGEOT 205 GTi |
1905 |
130 |
122 |
68 |
64 |
|
PEUGEOT 205 GTi |
1580 |
115 |
99 |
73 |
63 |
|
ROVER V8 |
3532 |
155 |
198 |
44 |
56 |
|
PORSCHE 911 |
3164 |
231 |
210 |
73 |
66 |
|
AVERAGE |
|
|
|
63 |
63 |
Although the average figures for both power and torque per litre
are almost the same there is a much bigger spread for the power figures. The
highest power output is 66% greater than the lowest whereas the torque per
litre figures only vary by 18%. We ought to have expected this because while it
is possible to tune an engine to deliver more power at high speed, there is
only so much air you can get into a cylinder per cycle which determines torque.
Let's see if the same story applies to 4 valve engines.
|
4 VALVE PER CYLINDER ENGINES |
|||||
|
ENGINE |
CAPACITY |
PEAK |
PEAK |
POWER |
TORQUE |
|
ROVER K SERIES |
1796 |
118 |
122 |
66 |
68 |
|
GOLF GTi |
1781 |
139 |
124 |
78 |
70 |
|
PEUGEOT M16 |
1905 |
160 |
133 |
84 |
70 |
|
HONDA VTEC |
1797 |
167 |
122 |
93 |
68 |
|
CITREON XSARA |
1998 |
167 |
145 |
84 |
73 |
|
BMW M3 SMG |
3201 |
321 |
258 |
100 |
81 |
|
AVERAGE |
|
|
|
84 |
72 |
Although both power and torque per litre are higher than for 2
valve engines we see a similar story with a much greater spread of power
outputs than torque outputs. In fact only the BMW stands out for its high
torque output (perhaps even a tad suspiciously so) although there is a 52%
spread of power per litre figures. We ought by now to be realising that
increasing torque per litre is much harder to do than increasing power.
In fact torque per litre figures can be used as a very good guide
to the truth or otherwise of quoted power claims. It is hard to get even a race
2 valve engine to produce much more than 75 to 78 ft lbs per litre and for
a 4 valve engine more than 85 to 88 ft lbs per litre. For modified road
engines though, especially those retaining standard type carbs or fuel
injection systems, the limits above are a good target. For big budget engines
where a lot of time and money has been spent on dyno testing of inlet and
exhaust manifold lengths and diameters then of course it is possible to push
the limits higher. With well developed cylinder heads, good inductions systems
(i.e. sidedraft carbs or even better, multi butterfly throttle body systems) and
efficient camshafts it is possible to push highly modified road engines to
around 80 ft lbs per litre for 2 valve designs and low 90s ft lbs per litre for
4 valve engines.