138 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Figure 6-27. This is a variable compression ratio engine. The cylinder head-cylinder assembly is mounted on pivot point. A mem- brane seals the gap between the block and cylinder head. A—In low-load operation, the compression ratio is high. B—During high-load operation, the cylinder head tilts (in this design), decreasing the compression ratio and allowing more supercharger boost. (Saab) A B Figure 6-28. This engine uses small hydraulic pistons acting on the rocker arm to alter valve lift and duration. (Honda) electro-hydraulically actuated arm. The arm can be raised or lowered to tilt the cylinder walls, cylinder head, and related top end parts in relation to the crankshaft centerline. A flexible membrane between the block and the cylinder head seals the crankcase. The crankshaft is mounted in the cylinder block in a conventional manner. During operation, the head and cylinders are held in the down position to increase the engine’s compression ratio for low-load, low-emission, high-fuel-economy operation. The supercharger is also turned off. During high-load operation, the assembly is lifted up to move the head away from the crankshaft. This increases the area of the combustion cham- ber and reduces the compression ratio. The supercharger is turned on, which boosts the pressure in the cylinder and increases the horsepower output. However, fuel economy decreases and emissions increase. An on-board computer matches the compression ratio to the load on the engine. In this way, a small-displacement, variable compression ratio engine can generate more power and better fuel economy than a larger, conventional engine. Variable Valve Timing Engine A variable valve timing engine (VVTE) can alter valve opening and closing independent of crankshaft rotation. These engines can match valve lift and duration to engine speed and load for improved power and lower exhaust emis- sions. Valve lift is how far the valve opens. Valve duration is how long the valve stays open. This improves engine efficiency compared to conventional engines that fix valve action with each degree of crankshaft rotation and single camshaft lobe profile. Most variable valve timing designs have electro-hydraulically operated rocker arms to alter valve operation. See Figure 6-28. A small hydraulic piston in the rocker arm shaft is used to engage and disengage the assembly. A rotating camshaft gear can also be used to change valve timing, but not lift and duration. At low engine speeds, only one intake valve is fully opened per cylinder. The other intake rocker arm for that cylinder only slightly opens its intake valve. This slight opening is needed to avoid fuel accumulation behind the valve face. By only opening one of the two intake valves, air velocity through the intake port is increased. This, in turn, improves atomization and mixing of the fuel charge for improved combustion efficiency at low engine speeds. At high engine speeds, the two intake rocker arms are locked to a mid-intake rocker arm that provides very high valve lift and longer duration. The mid-rocker arm rides on a high-speed camshaft lobe. This allows a larger, more- powerful air-fuel charge to enter the combustion chambers.