汽车专业英语Unit 6.pptx

Contents;Automotive English;Which parts of the engine can be performed without removing the engine from the vehicle ?;This Nissan engine takes the camshaft drive off the front of the crank using a single roller chain .;Geared camshaft drives use a direct connection between the crankshaft and the camshaft ;Timing belts are quieter than chains or gears, but require more frequent maintenance ;The timing belt on this transverse OHC engine is protected by a simple plastic shroud ;Automotive English; When you install timing chains, belts, or gears, all of the timing marks on the engine must be in alignment. The initial timing position for most engines is with number 1 cylinder at TDC of the compression stroke. There are exceptions. Some European engines ate timed with the number 4 cylinder at TDC of the compression stroke. ; Whenever you are going to remove timing components, it is a good practice to rotate the engine to align the timing marks as your first step. You want to avoid rotating the crankshaft or camshaft with the drive chain, belt, or gears removed. ; If you rotate an interference engine with the timing mechanism disconnected, there will be valve-to-valve or valve-to-piston contact. Contact of this nature can easily bend valve stems to create bigger problems. The engine will not run properly. Compression is lost because the valves cannot properly seat. Severe contact can also damage the piston crowns. ; If you must rotate the camshaft more than about 1/8-turn to position the timing marks, be sure to first rotate the crankshaft about 45 degrees off TDC in the normal direction of rotation (clockwise for most engines) to give the valves clearance for opening. Once the camshaft is in position, rotate the crankshaft backward to TDC and finish the job.;Timing chains are common in OHV engines. Chain drives are also used on some OHC engines, although composition rubber belts are also used. ;Some OHV engines use gear-to-gear setups to drive the camshaft; gear sets