VVT stands for variable valve timing, VVA for variable valve actuation.
If looked into an internal combustion engine the importance of the valve will be easily understood, as it facilitates the gas exchange process. the pistons drives the crankshaft and the crankshaft drives the camshaft, which in turn determines the opening and closing of valves. The gas exchange process is of paramount importance because it is the fuel that gets burned to give out energy.
At low RPMs the fuel is wasted and at higher loads it would be desirable if some extra charge can be provided. So it needs no genius to figure it out that if the gas exchange process can be effectively controlled a lot of improvement in fuel economy and peak power can be achieved.
For example, valve overlap is desired for race engines so that the burnt gasses maybe flushed out and the cylinder is filled with fresh charge, but valve overlap is not advisable for low loads because the engine can use internal EGR to save fuel at sub-optimal loads.
If delved deeper into the CAM of the camshaft, it comes out that there are 3 parameters which define a cam operation namely
- lift: how much the valve open (ranges between 5 to 8 mm for passenger cars)
- Phase: when the valve opens (ranges between 5 to 30 degrees before TDC)
- Duration: for how many degrees does the valve is kept open (200 to 270 degrees of crankshaft rotation.
After NISSAN rolled out its first car equipped with variable valve timing, all the companies have jumped in the bandwagon and currently all the automobile companies have developed one for their themselves namely VTEC, Vario-CAM, i-VVT.
All the famous valve timing systems would be discussed in detail in subsequent posts.
The requirement of torque in running an automobile is not linear, as the static friction is more potent that dynamic friction getting a vehicle in motion needs more amount of torque than to keep it running. the engine is not capable of varying the torque delivered at the drive shaft, it can only control the power with respect to the throttle.
The above graph depicts the torque-vs-RPM for different gears. It can be easily deduced from the graph that the lower gears give higher torque but lower RPMs. this is because Power is a product of torque and speed, one can be enhanced only at the expense of the other.
The power developed by an engine is constant for a particular throttle position. So in order to get more Torque (starting torque), a lower gear is engaged. For climbing to higher speeds the torque delivery is not the primary factor, its the rotation of the drive shaft, so the higher gear is engaged.
After getting a grip of why it is done, let’s delve into how it is done. The manual gearbox has two shafts each having a set of gears engaged with the other shaft. One is called the main shaft or drive shaft other is called the layshaft or the driven shaft. As engaging and disengaging the running gears causes undue stress on the gear tooth, constant mesh gearbox is universally used now. In the constant mesh gearbox, all the gears are in mesh and are always in motion. The dog clutch is connected to the gear shifter, which selects which gear combination would be giving the drive to the main shaft.
Now would be a good time to be introduced to the concept of overdrive, the gears have constant gear ratios which drive the wheels at a particular speed. the speed of the engines are limited, 5000-6000 RPM for petrol engines and 3000 RPM for diesel engines. At the top gear, most of the passenger cars have direct drive, meaning the main shaft runs with the speed of layshaft. But in the case of top-end cars such as race cars and sports vehicles the RPM of direct connection might not be satisfactorily high, so for those special vehicles, an additional gear is provided. that additional gear is called overdrive as that increases the speed of the driven shaft more than the driving shaft.
Turbochargers and superchargers are two cousins used for power enhancement. they both improve the performance of the car but do it in different ways. this would be elucidated as we delve deeper into them.
Turbocharger: These are basically a turbine which is provided at the exhaust manifold. it uses the kinetic energy of the burnt charge and boost the charging pressure of the intake manifold. Using the turbocharger the volumetric efficiency of an engine can cross 100% which is really a boon for performance engines. the increase in pressure which is normally calls ‘BOOST’ is the increase in pressure of the cylinder at the end of the intake stroke. The interesting thing to note about the turbocharger is that the RPM of the turbine can go ridiculously high up to the tune of 200000 RPM.
Supercharger: These are also turbines that use the crankshaft rotation to rotate the turbine on the intake manifold.As the turbine draws power directly from the crankshaft, it has a parasitic effect on the engine. As the crankshaft is rotating the turbine the RPMs don’t go to exorbitant levels, the max it spools is to the range of around 50000 RPM. and because the rotation of the turbine is lower than the turbine of Supercharger, the boost pressure is also lower.
From this preliminary investigation it comes out that the turbocharger is a sure winner, so what’s the point of using a supercharger which has a parasitic effect on the engine and even then doesn’t provide as high boost?
The answer to this question lies deeper in the cause and effects of having either systems.
The turbocharger surely uses the kinetic energy of the exhaust gas which would anyway get dumped, but that comes at a cost, the cost it pays is the back pressure generated because of the obstruction of the exhaust flow. on the other hand, the supercharger does have a parasitic effect on the engine but the increase in power it provides well compensates for the losses.
One more point of consideration is the charging mechanism of the engines.
Supercharger would definitely be better for petrol engines for 2 reasons
a) it runs at a higher RPM
b) The amount of charge depends on the load on the engine.
Turbocharger would be better for Diesel engines because the mass flow rate of the gasses (mostly air) remains the same for all the loads (its only the amount f fuel injected that matters).