For all you mechanics out there

Mate, finished my trade (and left) over 15 years ago. Cannot remember the term "Calorific value" is it like a "skirting board ladder"? It may have something to do with how many calories it burns?????????? :)
If I remember rightly Dwell is to do with the timing of the distributor and the angle of when the points open before top dead centre......
Thermal efficiency is the optimum operating temprature for the fuel to ignite the oxygen in the cylinder..... Engines will run more efficient in cold weather due to the density of oxygen. I think????
And Volumetric efficiency is another i am not to sure on. Probably a mechanical engineer question?????

Volumetric efficiency is the amount of fuel/air mixture that can be squirted into the cylinder before the valves close compared to the volume of the cylinder. Some old burnt gases still remain in the cylinder after the power stroke so the 'new' charge of petrol and air can not equal the actual volume of the cylinder so it will never be 100%

Do a Google search on the others as it will take to long to fully explain here.

Dusting off the text book’s hear LOL

Volumetric efficiency> (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, super chargers and turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE

Thermal efficiency> is the percentage of energy that is transformed (in work). For example, when 1000 J of thermal energy is transformed in 300 J of mechanical energy (remaining 700 J as heat), the thermal efficiency will be 30%. Its maximum possible value is known as thermodynamic efficiency. The thermal efficiency of a heat engine is the efficiency in which the chemical energy of a fuel is turned into useful work. It can be determined by dividing the useful work by the higher heating value of the fuel. It is a number smaller than 100%.

Calorific Value>
Amount of heat generated by a given mass of fuel when it is completely burned. It is measured in joules per kilogram

Dwell is actually the amount of angle of distributor shaft rotation that the points are closed which allows the primary winding voltage (6 or 12V) in the coil to build up and when the points open this magnetic field collapses over the seconday winding inducing anything from 20 to 50 thousand volts which is directed to the correct spark plug depending on the amount of cylinders and firing order. The primary winding is about 300-350 turns of heavy (about 1mm dia) wire and the secondary is about 20-30000 turns of wire about as fine as a hair (on your head-the other ones are thicker) So if the engine has 8 cylinders each lobe on the distributor shaft has a total of 45 degrees and about half of that the points are closed so the dwell angle would be about 22-24 degrees. If you imagine an 8 cylinder engine doing 5000rpm and 4 firing strokes every crankshaft rotation ie 4 every 10th of a second approx you can see why some V8's had two sets of points. Dwell angle is a much more accuarate way to set points than feeler gauges because you are not measuring the "mountain" that builds up on the negative side of a set of points (on a negative earthed system) I finished my time 33 years ago so I must have been awake during that lesson. If you try and repeat all this down at the local do it early otherwise you might give yourself away.

Power overlap is what I assume to be valve overlap, expressed in crankshaft degrees. This the period of how long both inlet and exhaust valves are open in the exhaust and intake stroke.
To explain this imangine that a a cylinder is at the firing point of the compression stroke. The piston is forced down and before bottom dead centre (btc) the exhaust valve will open. Exhaust gases are then going into the exhaust port / manifold. As the piston rises on the exhaust stroke the process is sped up.
As the piston nears top dead centre (tdc) the inlet valve opens.The escaping exhaust gas draws the petrol / air mixture into the exhaust port/ manilold. When the piston changes direction and starts it`s inlet stroke it draws the mixture from the exhaust port / manifold and also from the inlet manifold. At a certain point the exhaust valve will close and only mixture is then drawn from the inlet manifold .
This helps cylinder filling, the vortex effect of the exhaust gases starts the filling process of the cylinder.
The camshaft is obviously the key to all this. A B.M.C. Mini full race camshaft has 80 degrees overlap. The more overlap the less power the motor has at low speed. The motor will not idle smoothly and power can not be achieved until higher rpm.
My Mini had a full race camshaft and a side draught Weber carburettor. It would not idle well below 1200 / 1300 rpm. Also full power did not come in until 3500 / 4000 rpm. Hope this description is understandable. Regards G.T.
P/S - all cars have some overlap but in a far less radical manner than competion cars.