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By Guy Croft BSc
Part 2: Volumetric efficiency
You will have noted in the dyno plot in Part 1, the marked difference between the power curve, which rises quite linearly, and the torque curve - which climbs, levels off and then falls. This is typical of normally aspirated reciprocating gasoline engines. The engine power will climb until a point is reached where the torque is so poor that raising the speed even higher (in the case of a dyno test by reducing the load with the throttle wide open) leads to ever decreasing power output.
According the equation shown in Part 1, P = 2 x Pi x N x T. Power depends on Torque and engine speed. We always want to develop as much â€¹Ã…â€œarea under the torque curve as we can get, this means getting plenty of air/fuel into the cylinder during the intake stroke, in other words - the ability of the engine to â€¹Ã…â€œbreatheÂ¢Å¾Â¢. The achievement limits of the torque-power-speed curves are of course also determined to a great extent by the mechanical strength of the engine components (rods, pistons, crank, flywheel, clutch, valve springs etc). What is notable of course in the case of the 2 liter Fiat engine cited in Part 1 is how the engine holds its power over 7000rpm. This engine was built to give a working power band from 4000 to 7500 rpm. The flat power curve at the top end of the power curve is a sign that the engine, far from struggling to inhale air at the higher speeds, is breathing very well indeed. It must be - or the torque would drop off severely and take the power with it. In other words the inlet tract, cams and valve train, exhaust system are interacting well.
As far as modifying production engines goes, the job for a good race engine builder is to maximise the torque output throughout as broad a range of engine speeds as possible. To increase torque and raise the engine speed at which it can be developed we need to get more fuel in and Â¢â€šÂ¬Ã…â€œ importantly Â¢â€šÂ¬Ã…â€œ burn it faster and more effectively.
In summary Â¢â€šÂ¬Ã…â€œ we need the following attributes in our competition engine:
- high volumetric efficiency (Vr)
- fast burn rate
- higher compression ratio (CR)
Of these higher Vr is by far the most significant Â¢â€šÂ¬Ã…â€œ we can sacrifice the first two within limits, but without superior airflow into the head and adequate cylinder purging after the exhaust stroke the engine will never be optimised. It is the primary influence on torque Â¢â€šÂ¬Ã…â€œ and its characteristics mainly from the head and associated hardware.
Vr is defined by the ratio:
Actual mass of air induced into cylinder on one cycle
Mass of air to fill swept volume of cylinder at atmospheric pressure
Vr is principally controlled by the throttle position Â¢â€šÂ¬Ã…â€œ this is after all the main means of controlling the engine output. Control of speed by throttling is highly inefficient because it generates a pumping loss, wherein the engine has to suck the mixture into the cylinder past the throttle at part load operation.
Vr varies throughout the engine speed range, creating an uneven and non-linear torque response. Because of this the signature power curve of every engine is different. Air is a uniquely slippery and unpredictable medium, but with careful tuning it can be caused to build up momentum at the right time, generating column-inertia that can ram-charge the cylinder on the intake stroke and produce a useful â€¹Ã…â€œhumpÂ¢Å¾Â¢ in the power curve.
Vr is affected by almost everything else around it, for example: pressure waves (some useful, some not) across the inlet and exhaust ports - as the valves open and close (themselves dependent on the port lengths and sizes) , the effectiveness of cylinder purging of residuals on the exhaust stroke, the potential of the inlet tract to flow air, the valve size and camshaft lift/timing characteristics, air density, ignition timing, fuel distribution.
- The improved flow of the big valve head over standard can yield vastly better Vr & more torque - but may need a more aggressive cam adn high CR. This is a SOHC 1300 Fiat Uno head.
- Big valve SOHC 42mm inlet valve.jpg (129.52 KiB) Viewed 18886 times
- SOHC Fiat - go too big on the ports on any engine with a weak cam setup and low CR and you have can cause a 'mismatch' - poor cylinder filling = low Vr and very poor torque.
- JV X19 port comparison to std.JPG (42.06 KiB) Viewed 18872 times
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