VVT enables high-speed efficiency or low-speed power. At the same time, cylinder #3 might be reaching TDC on the compression stroke, so the camshaft would leave those valves closed.Ĭamshafts fitted with variable valve timing (VVT) use hydraulic actuators to advance or retard valve timing in relation to crankshaft angle. For example, as cylinder #1 is coming to top dead center (TDC) on the exhaust stroke, the camshaft is opening the intake valves and closing the exhaust valves. Valves must open and close at specific intervals in relation to cylinder position. These camshaft hydraulic phasers effect variable valve timing on intake valves and exhaust valves. This works well on average but doesn’t excel in any one driving situation. This is referred to as valve opening duration. Also, a fixed camshaft might open the intake valve 10° before TDC (BTDC) and close it 5° after bottom dead center (ABDC) and open the exhaust valve 15° before bottom dead center (BBDC) and close it 5° ATDC. On fixed camshafts, this is not adjustable, but there are circumstances at which the engine might “breathe” better if only the valves could open a little more. Similarly, valve “lift” refers to the height of the lobe in relation to the center of the shaft, which determines how far the valve opens. The camshaft has four distinct strokes: intake, compression, power, and exhaust.Ĭommon camshafts are made to match typical operating characteristics and may accentuate highway cruising efficiency or low-end power. The camshaft gear has twice as many teeth as the crankshaft gear, which allows it to rotate at half the speed of the crankshaft. In some pushrod engines, timing gears may also be used. The camshaft is usually connected to the crankshaft using a timing chain or timing belt. As the cam lobe rotates further, the valve spring forces the valve back up, closing it. Similarly, in SOHC and pushrod engines, the cam lobe pushes on the rocker arms (or pushrods then rocker arms), opening the valve. In DOHC engines, every rotation causes a single cam lobe to push the valve down, opening it into the cylinder. Some performance and custom camshafts may be milled from a solid steel block.Īs the camshaft rotates, the cam lobes move up and down. The typical camshaft is milled from a rough-formed cast steel blank. Pushrod engines usually have two or three valves per cylinder, usually one intake valve and one exhaust valve. Long metal pushrods transmit camshaft movement to the rocker arms, which transmit that movement to the valves. Older engines and a few newer “pushrod” engines have a single camshaft in the cylinder block. Overhead-cam engines have three to five valves per cylinder, but usually two intake valves and two exhaust valves. An i4 DOHC engine has two camshafts, while a V6 or V8 DOHC engine has four camshafts. An i4 (four-cylinder) SOHC engine has one camshaft, while a V6 or V8 SOHC engine has two. Force is transmitted directly to the valve. Dual OHC (DOHC) engines have two cams per bank, usually directly over the valve stems, one for intake valves and one for exhaust valves. Rocker arms transmit SOHC movement to the valves. Single OHC (SOHC) engines have one cam per bank, usually mounted between the valve stems. In modern overhead-cam (OHC) engines, the camshaft is located in the cylinder head. ThyssenKrupp Presta Chemnitz GmbH / Wikimedia Commons A collection of camshafts from the engines of cars and trucks.
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