Mind Blogging. Mind Boggling.

If you ask me what i'm doing now, I would say I'm in third year of Mechanical Engineering. However, my blog, up until today, doesn't give the beholder the faintest whiff of this dominant preoccupation of mine, so i decided to set things right. After football talk and book reviews and movie gossip, here I am, writing a piece on torque / power chars. of a car and camshaft technology- finally a post befitting the title of a mechanical engineer which i lay claim to !




I have long been befuddled by the power torque curve of an automobile, which is mentioned ever so often in automotive parlance. And after some digging up in the corners of my vast brains and the corners of the vast internet, i have come up with some satisfactory answers.

We should remember that

work = force * displacement.

So if you slide a block along a smooth surface, you might be applying very little force, however the dispacement you produce will be high. So the net result is a high work done. However, consider a block ona very rough surface. Even if you apply ten times the force you did in the earlier case, your displacement might be neligible, and the net effect would be that your work would be lower than the former case.

Now power is rate of doing work, so the same arguments in the above para could be extended to power- you impart higher velocity on the smooth surface, so lower force could see you producing a higher power.

This is exactly what happens in the automobile. The torque curve reaches its peak much before that of the power curve. However, power = torque * rpm, hence the power keeps increasing and stats falling off much after the torque peak, as seen in the pic. Now the question is, why does torque start falling off after a point, when anyway your carburetion system delivers fuel proportional to rpm, more fuel at more rpm's . One would expect torque to be just linearly incrasing with rpm, as you are increasing the fuel supply with rpm.

The point is, the more air fuel mixture you pack in your cylinder, the more torque you get out of the engine. At higher rpm's the valves open and close very fast. So the air doesn't have enough time to get into the cylinder. Result- less air in the cylinder, less fuel fuel burning, less torque. So what one would like to do is also increase the time the inlet valves are open- at higher rpm, leave the vlaves open for a longer time. Obviosly, there are limits- you can't leave the inlet valve open for much of the exhaust stroke, otherwise the exhaust gases would be pushed into the inlet manifold and prevent fresh mixture from entering. However, tweaking is always possible- you can change not only the timing, but also open the valve more, or give it more 'lift', so that more air can squeeze in.





CAMSHAFT OPERATION (image courtesy howstuffworks.com)

This is what Honda has done with its VTEC ( Variable Valve Timing and Lift Electronic Control) The camshaft has diff. profiles for low and high rpm operations of the engine- based on rpm and load, the onboard computer selects the lobe which it wants active. All manufacturers have a variable valve timing system, which are often given fancy acronyms- Toyota's VVTi (Variable Valve Timing with intelligence), BMW's VANOS (Variable Nockenwellen Steuerung) , GM's DCVCP (Double Continuous Variable Cam Phasing) , Porsche's Vario Cam ( Which adjusts tension between the chain connecting intake and exhaust camshafts) .

An intersesting variant is BMW's Valvetronic system. They rely on amount of valve lift to throttle the engine, rather than the traditional butterfly valve in the inlet manifold. The butterly valve is kept at a default open position throughout, closing only during idling.

So there ends my technical article. Hope the article drove the point home- camshaft and valve timing technology is a big field today- indeed the cam era is here !