The events that define that combustion, such as how long the valves remain open, occur over even smaller fractions of a second. And as the engine approaches its rpm climax, the entire process is compressed into a window that lasts roughly one-tenth as long as it does at idle. The dwell translates to a total opening of 0. It can adjust intake-valve lift between 0. Exhaust-valve timing: By controlling the camshaft timing independently, the engine controller can adjust the amount of overlap—the period when both the exhaust and intake valves are open.
At low-load, steady-speed cruising, this overlap is increased to allow a portion of the inert exhaust gas to flow back into the cylinder during the intake stroke, reducing combustion temperatures and the formation of oxides of nitrogen.
Recover your password. Friday, November 12, Get help. The GoMechanic Blog. But without going into the detail of VVT we first need to look at what is valve timing. And the valves are like the doors for the ports from which the air or air-fuel mixture enters from the intake and exits the chamber from the exhaust valve when it opens.
As said earlier the cam or the lobes on the camshaft control the opening and closing of the valves. They ride the valves at definite intervals and with utmost extreme precision. Defining the valve timing of an engine is one of the most typical processes.
Firstly, the inlet valve opens to let the air or air-fuel mixture enter the combustion chamber. This valve opens a few degrees before the piston reached TDC.
This is when the engine is about to complete its exhaust stroke. The inlet valve closes after the piston reaches tad bit further than the BDC. Also called the suction stroke. The inlet valve closes just before the compression stroke. This is when both the valves are shut inlet as well as the exhaust valve. If the engine is FI Fuel Injected , the fuel is atomized sprayed into tiny droplets inside the combustion chamber before the piston reaches TDC and then fuel is ignited in SI engine Spark Ignition engine.
This also depends on the ignition timing. Coming back to the valve timing, the exhaust valve opens way before the piston reaches the BDC after expansion stroke or power stroke. Blow Down Both the valves remain shut to perform the combustion process efficiently during the compression stroke all the way up to the power or expansion stroke.
Overlap In the diagram, it is very prominent that when the piston reaches TDC in the exhaust stroke, both the valves, intake and exhaust, are open. Not doing so can cause some burnt exhaust gasses to remain inside the combustion chamber and dilute the air-fuel mixture Ram Effect This is the situation where the intake valve closes at a few degrees after the BDC. So the engineers have to tune the valve timing with less overlap.
This is to reduce the escaping of the fresh charge during the suction stroke. On the other hand, a racing motorcycle or car has to run on high speeds and the engine has to keep on the boil. Hence, the intake and the exhaust valve has to have an increased overlap period. As said earlier, it helps in the suction of fresh charge from the intake manifold at high revolution. The third situation is the road cars. This is where the car run on both, in the city and out on the highway. So, performance is best in the middle of the rev range.
Hence the city cars have their valve timing calculated in these conditions in mind. Variable Valve Timing By now you might have an idea what the variable valve timing would be. In simple words, a traditional engine with no VVT operated on only a single cam profile.
Before we look at VVT-i, let's talk about sensors. VVT systems use all sorts of sensors, but the most important are the camshaft and crankshaft position sensors which are often hall effect sensors. The crankshaft is connected to the rod and piston, and the camshaft's lobes trigger valve lift events.
Thus, with the information from the crankshaft and camshaft position sensors, the ECU can learn how fast the engine is rotating and the relative positions of the piston and the intake and exhaust valves. Cam phasing advances or retards valve lift events by rotating the camshaft, typically over a range of about 60 degrees relative to the crankshaft angle.
Let's say our intake valve normally opens 5 crankshaft degrees before top dead center and closes crankshaft degrees after top dead center 5 degrees after bottom dead center. There are numerous different methodologies used to vary valve timing. Each manufacturer has its own name for its own VVT system. In this system, the ECU receives signals from the camshaft position sensor, crankshaft sensor, oil temperature sensor, mass air flow sensor MAF , and the engine coolant temperature sensor and uses the information to adjust its output signal to an oil control valve.
This valve acts as a hydraulic actuator, rotating a rotor which is connected to the camshaft inside a housing, which is connected to the crankshaft via a timing chain. Once the ECU has changed the cam phase angle, the ECU continues to receive inputs from all of the sensors and continually adjusts the oil feed to the rotor. Like electronic throttle control , this is a closed loop system, which means that the difference between the current camshaft phase angle and the optimal camshaft angle is the "error signal" that is sent to the ECU.
The computer uses the error signal to adjust its output to the actuator to get the camshaft phase angle where it needs to be. Other VVT systems change the shape of their camshaft lobes, not just the camshaft phase angle relative to the crankshaft. Changing the cam profile not only affects valve lift how far the valve opens , but also valve duration how long the valve stays open. The image above demonstrates the features of a camshaft lobe that affect valve lift and duration. At higher engine speeds, many VVT systems change to a more aggressive i.
Some variable valve lift systems shift the camshaft axially so that a higher profile lobe engages the cam follower, producing more valve lift. Others, like Honda's VTEC yo , lock a high profile rocker arm to the low speed rocker arms via a hydraulically actuated pin.
A more aggressive cam lobe activates this high profile rocker arm and provides the intake valve with more lift, allowing more air into the cylinder.
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