Saturday, April 08, 2006

Intake Horns - an excerpt from the draft of 'Automotive Engines'

I have gone back to writing my book. Yes, it sounds grand doesn't it?
Writing on my book.

Just remember that many books get written. A large quantity can never meet the standards required of publication. Some of those published never make it anywhere significant (the rubbish tip is somewhere, but...).

Anyway, here's a first draft of my latest section, "Intake Horns", from the chapter "Induction and Exhaust".

As this is a draft, I would be very happy if errors and omissions of any sort are pointed out. Thank you.


Intake Horns
An excerpt from the draft version of 'Automotive Engines' by Tan Yee Wei

Induction manifold: a branching pipe that delivers air and fuel to individual cylinders.

In many engines, intake air is filtered to remove dust and grit that may harm the engine, then piped towards the carburettor or fuel injectors, then diverted towards the engine block. Here, the pipe splits into smaller pipes, each delivering air and fuel to individual cylinders. This is when the induction manifold is relevant.

However, some extremely high performance and short race duration engines can make do without an air filter. It is also likely that this engine will have individual fuel injectors at each cylinder. Thus there is no need for the intake manifold which only serves to distribute air that has been modified by a central facility, namely the air filter and fuel injector.

In these cases, the induction ports of each individual cylinder are equipped with a pipe that widens at the opening in a horn shape to improve airflow. For obvious reasons, these are called intake horns.

These extremely high performance engines not only gain an advantage in not having to draw air through an air filter, they also do not have to draw air through long lengths of piping. The upside is less energy required to take air into the cylinder.

This advantage can also be applied to engines that require an air filter (but still retaining individual fuel injectors at the cylinders). The entire collection of intake horns can be enclosed in an air-tight box, which is connected to the air filter by a sufficiently large pipe. Thus the intake plenum is born, also referred to as the air box. The advantage of this arrangement is that the connection between the plenum and the air filter can be made arbitrarily large to reduce resistance, and the overall resistance in the intake system can be reduced significantly by taking the manifold out of the picture. In some cases, the air box also doubles as the air filter: the walls of the air box are made of air filter material, thus doing away with the extra piping.

The benefits do not stop there: these induction horns can be easily tuned to optimise airflow at certain speeds. This can be done by simply changing a set of horns for a longer or shorted set. Performing a similar operation with an intake manifold would be a terribly convoluted exercise, in all senses of the word.

In a competition engine, one might expect the horns to be tuned to perform best when the engine is producing its peak power, since the engine will be operating near its peak power most of the time. However, things might differ in the case of a passenger vehicle’s engine that is expected to operate at various speeds. The horns may be tuned to give the extra bit of power where the engine is lacking in power, so as to give it decent performance all round its operating speeds.

The major point of consideration in tuning the intake horns is the length of the horn. In general, longer horns give good performance at low engine speeds, and short horns give positive results at high speeds.

A simplified explanation of induction tuning is presented below.

Consider one cylinder of an engine with intake horns installed. At this moment, it is in the midst of the intake stroke- the intake valves are open and the piston is moving downwards, drawing air into the cylinder through the intake valves. Air in the intake horn is moving inwards towards the engine.

When the intake stroke is complete, the intake valves close. At the instant the valves close, air in the intake horn is still moving inwards. We would expect that this bulk of air will be stopped, since it will not get past the closed valve. Thus the air piles up against the closed valve- its velocity slows to zero, while the pressure and density increases.

This pressure build up then starts to push the air backwards out of the intake horn, the backflow only stopping when the excess pressure is dissipated.

This is the point when induction tuning becomes relevant. The length of the induction horn is adjusted such that the duration of the pressure increase corresponds with the time it takes for the engine to complete one cycle (2 revolutions). Thus at the time the pressure is ideal, the intake valves are already open for the next cycle of air intake.

The pressure and density build up thus aids to push air into the combustion chamber. Of course, one may see that if the engine speed is slower than ideal, the air would be already moving backwards by the time the valves open for the next cycle. The result is an engine that gives good performance only in a narrow range.

The next question would obviously be directed in the general direction of giving excellent performance at all engine speeds. In the upper echelons of high-tech, big money motor sports, variable length intake horns are used. Prior to 2006, variable length intake horns were used in Formula 1 engines. This naturally gave spot-on induction and good power at all engine speeds.

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