VARIABLE VALVE TIMING

Until recently, a manufacturer used one or more camshafts (plus some pushrods, lifters and rocker arms) to open and close an engine's valves. The camshaft/camshafts was turned by a timing chain that connected to the crankshaft. As engine rpm's rose and fell, the crankshaft and camshaft would turn faster or slower to keep valve timing relatively close to what was needed for engine operation. Unfortunately, the dynamics of airflow through a combustion chamber change radically between 2,000 rpm and 6,000 rpm. Despite the manufacturer's best efforts, there was just no way to maximize valve timing for high and low rpm with a simple crankshaft-driven valve train. Instead, engineers had to develop a "compromise" system that would allow an engine to start and run when pulling out of the driveway but also allow for strong acceleration and highway cruising at 70+ mph. Obviously, they were successful. However, because of the "compromise" nature of standard valve train systems, few engines were ever in their "sweet zone," which resulted in wasted fuel and reduced performance. Variable valve timing has changed all that. By coming up with a way to alter valve timing between high and low rpm's, Honda, Toyota and BMW and many more manufacturer's can now tune valve operation for optimum performance and efficiency throughout the entire rev range.

INTRODUCTION

Engine breathing is analogous to the breathing of any living organism. At rest, the lungs take in the necessary amount of air for normal function. When running, the lungs and heart work faster to supply more oxygen to the system. Engines can't do that because their breathing apparatus (comprised intake manifolds, intake runners, valves, valve lift and throttle bores) is fixed.

There was a time when engines had to be big to be powerful. There was a time when engines could either be tuned for low-rpm torque or high-rpm power, but not both. There was a time was a time when a specific output of 100 hp per liter was the stuff of racecar fantasies. Today these limitations are all but gone. Getting 100 hp for each liter of displacement is now possible on cars that have to get good gas mileage, emit clean air, act civilized enough for your grandmother to drive them and sell for under $20,000.Remember that an engine is basically a glorified air pump and, as such, the most effective way to increase horsepower and/or efficiency is to increase an engine's ability to process air. There are a number of ways to do this that range from altering the exhaust system to upgrading the fuel system to installing a less-restrictive air filter. Since an engine's valves play a major role in how air gets in and out of the combustion chamber, it makes sense to focus on them when looking to increase horsepower and efficiency. This is exactly what Honda, Toyota and BMW and quite a number of other manufacturer's have done in recent years.

Popet values are used in gasoline and diesel engines to control the inlet and exhaust of air passing through the engine. When the intake values open, air is drawn into the engine cylinder. After the fuel has been burnt, the exhaust valves then open to let it leave. In conventional engines, the popet valves open and close at a constant speed. Their timings do not depend on how fast the engine is running. At high engine speeds [e.g. when overtaking a slower vehicle], this starts to become a problem. Large amounts of air are required by the engine at higher speeds. However, the intake valves may close before all the air has been given a chance to flow in. On the other hand, if the valves were calibrated to remain open for longer periods of time, problems start to occur at the lower engine speeds. In these situations, unburnt fuel may exit from the engine since the valves are still open. This leads to lower engine performance and increased emissions. By using advanced systems to alter the opening and closing of engine valves, they have created more powerful and clean burning engines that require less fuel and are relatively small in displacement.

BENEFITS OF VVT

Smooth Idle

At idle rpm, retarding the camshaft eliminates valve overlap. With the intake valve opening after the exhaust valve has closed, there is no blow back of exhaust gases to the intake side. Now, combustion is more stable because of the clean air/fuel mixture. This allows the engine idle smoothly at a lower rpm and fuel consumption is reduced.

Torque Improvement in Low to Medium Speed Range

In the low to medium speed range with a heavy load, the camshaft is advanced increasing the valve overlap. This has two effects. First, the exhaust gases help pull in the intake mixture. Second, by closing the intake valve early, the air/fuel mixture taken into the cylinder is not discharged. This improves volumetric efficiency and increases torque (and therefore horsepower) in the low and midrange rpm range. The driver notices a more powerful acceleration. Fig. 8 shows early measures of the VVT mechanism intake cam torque (friction) values. The cam torque for low lift is less than a baseline for low speed and low lift, but greater for high lifts and high speeds. Figure displays the effects of VVT upon idle speed operation.

EGR Effect

VVT eliminates the need for an EGR valve. As a result of increasing the valve overlap in which the exhaust and intake valves are both open, the exhaust gas is able to flow to the intake side. Diluting the air/fuel mixture with exhaust gases reduces the combustion temperature and the production of NOx. Also, some of the unburned air/fuel mixture present in the exhaust gas will be burned. Figure compares engine combustion stability (COV of IMEP) between the baseline and VVT configurations.

Better Fuel Economy

A VVT equipped engine is more efficient and provides better fuel economy from a variety of factors. Without VVT, the engine would have to be larger and heavier to produce the same horsepower. Smaller pistons, connecting rods, and crankshaft reduce friction and mechanical losses. A lighter engine improves vehicle fuel economy.

Improved fuel consumption is also realized because of the further reduction in the intake stroke resistance. In the medium-load operation range, when the valve overlap is increased, the vacuum (negative pressure) in the intake manifold is reduced. Now, it takes less energy to move the piston downward on the intake stroke. With the pumping loss reduced during the intake stroke, more energy is available to propel the vehicle.

At idle, with no valve overlap, the idle speed is lower improving fuel economy. Figure shows that at 2000 rpm, the application of the VVT mechanism to this engine (without additional engine design changes) reduces the Brake Specific Fuel Consumption (BSFC) in the low to middle load regions by 5 – 7%.

CONCLUSION

So after this seminar we come to know that VVT technology is going to become prominent in road cars as this technology is going to be researched a lot and may be in few years its cost of production can be brought down to incredible levels. We already kwon about the benefits of such a system and thereshould not be anyone who dislikes this technology, since in today’s eco-friendly world this technology brings a ray of hope for a cleaner and healthier environment. Already the automobile giants like Honda, Toyota, BMW, Mercedes-Benz, etc. are spending lots of money on research in this field. So VVT technology is bound to have a bright future in the automobile industry.