Twin overhead camshafts

Twin overhead camshafts, we have all got them but what is the advantage of twin camshafts over single overhead camshafts or god forbid overhead valve layouts? The first thing to point out is that an overhead camshaft arrangement reduces the amount of mass that has to be moved by the camshaft and more importantly the less mass that has to be moved by a valve spring. The more weight in the valve train the stronger the valve spring has to be to move the valve within a certain time period and the stronger the valve spring the more power the camshaft uses to open the valve. There is also the issue of push rods flexing at high speed again the stronger the valve spring the more liable the push rod is to flexing which has a significant effect on valve timing.

Moving on from the simple issue of mass in the valve train to understand the relative merits of single overhead camshaft verses twin overhead camshafts you have to bear in mind that air has mass and anything that has mass has inertia. From Newtons 1st law which states:- In the absence of net external force, a body either is at rest or moves in a straight line with constant velocity. From this we can understand that both the incoming charge and the out going exhaust has inertia, it takes time to accelerate and decelerate. This does not at first seem important as to us the mass of air is very small however in the inlet tract of your Giulietta the air can be travelling at trans-sonic speeds and so the mass then becomes an issue.

Then you have to remember that this is not a steady flow it only flows for about ¼ of the engines cycle, so you have a slug of air travelling at around 600 mph hitting the back of the inlet valve and it does not want to stop so in compresses against the back of the inlet valve. Mean while on the exhaust side of the engine the exhaust valve is closing and the piston has sent its own slug of exhaust gas down the exhaust pipe, this also has momentum but the momentum of the gas is operating the other way round to create a vacuum. So the exhaust is creating a vacuum and the inlet manifold is creating pressure and so as soon as the inlet valve opens the inlet charge helps to clean out the remaining un-burnable exhaust gas from the combustion chamber. This is fine with a twin cam engine as the valves are opposite each other and so it works well but on a single overhead cam shaft engine the valves are staggered along the line of the gas flow and so the inlet charge does clean out the exhaust gasses as effectively and there for power is reduced.

Looking at the evolution of the Giulietta engine you can see how Alfa Romeo have used the dynamics of gas to best effect, you do have to remember that all the development of this engine was done by experience and trial and error, there wasn’t and huge computers doing finite element analysis here. On the Normale the inlet manifold is slightly curved as the charge is fed through a single carburettor while the Veloce has a single choke per cylinder feeding a straight inlet tract, which is perfect with regard to Newtons first law. There are other advantages as well with using a side draught carburettor as opposed to a single downdraught carburettor. Firstly the fuel is considerably heavier than air and so the fuel separates from the air as it tries to do the right angle under the carb and this wets the manifold, which is not a problem with a warm engine as it just evaporates again, however on a cold engine the fuel does not evaporate and you tend to need the choke, whereas with the side draught carbs on a Veloce you may only need three pumps of the throttle to get it going and once it is running it may only need a slight amount more throttle to keep it going rather than having to keep the choke out which adds more fuel to compensate for the fuel “lost” in the manifold by wetting.

Next the charge comes to the inlet valve this opens perhaps surprisingly while the piston is still coming up the bore. On the Normale the valve opens 22 degrees before the piston gets to top dead centre (TDC) while on the Veloce it opens the valve earlier, just over 25 degrees before the piston reaches TDC this allows the pressure in the inlet tract to be exploited to the full, you will note that the greater pressure in the Veloce’s inlet manifold allows you to open the valve earlier with out affecting drivability. Meanwhile on the exhaust side of the engine the exhaust valve is still open and on this stays open until 12 degrees after TDC on the Normale and just over 18 degrees on the Veloce this gives you what is known as valve overlap and it is usually a fair indicator of the “sportiness” of an engine, so a Normale has 34 degrees of overlap while the Veloce has 43.6 degrees of overlap. Meanwhile at the bottom of the stroke (BDC) the inlet valve stays open while the piston comes up the bore this allows the momentum of the air to continue to fill the bore even though the piston is coming up in the opposite direction. Meanwhile the exhaust valve also opens before the piston has reached the bottom of the power stroke to allow the spent exhaust gasses more time to escape. The time the valves are open is known as the duration of the camshaft and the inlet cam on a Normale has a duration of 267 degrees while the Veloce has a duration of 273.20 degrees again as with increased overlap a greater duration is usually allied to greater performance.

Alfa Romeo use a different exhaust on the Normale to the Veloce, once again this is to exploit the momentum of the gas. As the gas goes down the exhaust at around 180 psi its momentum first sucks out the remaining gas from the combustion chamber but then as it meets the next exhaust port it has a tendency to blow back up the other ports and cause problems with the filling of the next cylinder that opens its exhaust valve. To overcome this effect you will notice that on the Normale which has shorter exhaust stubs the exhaust valve closes earlier so that the back pressure from the next cylinder exhausting its gas has less effect on the charging of the cylinder next cylinder. The Veloce meanwhile has much longer primary exhaust tubes this helps stop the exhaust gasses from one cylinder bleeding back into its neighbour and so the valve timing can be more extreme without affecting drivability.

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