Time it well

We aren't talking Cricket here. So, naturally, it ain't about any batsman being able to time a shot well. There are many more places other than cricket where you have to time it well. Cars are no exception. No! it isn't about time taken for reflexes for car control either. So, what kind of timing are we really talking about? You often see cars wearing badges like VTEC, VTVT or VANOS and you would normally ignore them saying, "Huh! just another jargon". But, if you try to find out more about all these technologies, you'll know it's one or the other form of Variable Valve Timing or VVT for short. Let's spend a while in understanding this accessory of your car's engine. Well, it's not just an accessory per se. It is rather a very integral part of modern engines.

Let's first take a minute to brush up the basic (crude) working of an internal combustion engine. As you probably know, an engine comprises of one or more cylinders. Each cylinder has a piston and a set of valves. The piston completes two full up-and-down strokes for one complete power cycle (in 4-stroke engine). The first stroke in the cycle is "intake stroke". In this stroke, piston moves downwards opening the intake valve. This sucks in a charge of air mixed with fuel. In the second stroke, the "compression stroke", intake valve shuts and piston starts moving back up. Piston compresses the fuel and air mixture. Third in the cycle is "power stroke". The intake and exhaust valves are both closed, and piston is at the very top of the cylinder. Spark plug ignites the fuel and air mixture in the cylinder causing an explosion. The force from the explosion drives the piston down. In the fourth and final stroke, called "exhaust stroke", piston moves down all the way to the bottom of the cylinder and starts to head back up, exhaust valve opens releasing out the gases from explosion.

The valves we just talked about are called "Poppet Valves". They are opened at the appropriate time by one or more rotating camshafts (A piece of metal which translates the circular motion into a series of pulses through the cams protruding out from it. ) Cam lobes on the camshaft push the valves open just at the right time in the engine's operating cycle, then keep the valves open for just the right duration. You must have guessed by now that the timing of these valves could be a critical factor deciding the performance of your car's engine. And it indeed is. The cams are turned in precise relation to the crankshaft by chains, belts or gears—or a combination thereof. In the most primitive form, relative position (or timing) of the cams and crankshaft (Almost forgot to mention... for newbies, crankshaft is a rod attached to pistons which converts linear motion of pistons into circular motion) is fixed. This means that, in a given cylinder, intake valves are always opened or closed exactly at the same point in every stroke, regardless of how fast or slow the engine was running. A production form of this is achieved using something called as "cam phasing". Then there is this variant called continuous variable valve timing, in which, the lift (opening) of valves varies with the revs of your engine. This is done to improve the power delivery of your engine. A natural question here is , how? How does it improve power delivery?

Physics of air plays a vital role in the working of VVT. It's the inertia of air (due to its mass), that is, its reluctance to get into the combustion chamber (or cylinder) that makes a huge deviation in the amount of air entering at the lower revs than that at higher revs. This has fairly simple explanation, the time for which valves are open is significantly longer at lower revs compared to engine operating at higher revs. The same is true for exhaust (valves) as well. Therefore, there needs a way to accelerate air into and out of the engine cylinder; and make sure that the cylinder is filled or emptied correctly at all RPMs. This is exactly what VVT does by adapting with the engine speed, the valve opening (lift) and the time for which it stays open.

There are a few other variants of VVT. One of them is what we see on Hondas. Yes, Honda VTEC, an acronym you've seen on literally zillions of cars, primarily City. Honda's VTEC(Variable Valve Timing and Lift Electronic Control) technology actually makes use of two separate cams. Either of them kicks in at predefined RPM levels. You know, it ain't a very sophisticated technology when compared to continuous VVT. But its simplicity has worked for Honda pretty well. Honda, however , has now introduced a continuous variable variant of VTEC, called intelligent VTEC or iVTEC that goes onto our very own Accord. Every manufacturer has a different name for the same technology, like Hyundai calls it the VTVT (Variable Timing Valve Train) or BMW prefers calling it VANOS. Variable valve timing was first used in a production car by Fiat in 60's. In its most mature form today, it is still used by a Fiat group company; Ferrari employs state-of-the-art VVT. It features cam lobes that are tapered along the length of the camshaft. An electronically controlled system then moves the cam laterally in proportion to engine rpm, so that the effective cam timing, lift and duration can be varied.

Far from a single technology, VVT is a catchall title; it covers systems ranging from those that simply rotate the camshaft slightly to those that eliminate throttle bodies entirely by dynamically changing valve lift. But, how does this complex thing really help you and your car? Well, the advantages of VVT are many. Improvement in the power delivery, better torque and extremely important - better fuel economy to name a few. Recent Honda iVTEC commercial explains it in the best possible fashion. The tagline says it all, "Drive Every Drop".

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