LO 9-6 Identify and explain the purpose, construction, and operation of the camshaft
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 9-6 Identify and explain the purpose, construction, and operation of the camshaft |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:44 PM |
1. Camshafts
LO 9-06: Identify and explain the purpose, construction, and operation of the camshaft. |
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Camshafts Camshafts are critical components affecting engine performance because they control the gas dynamics or engine breathing characteristics Figure 47. They are commonly made from nodular cast iron. More recently, hollow camshaft tubes are made with eccentric lobes that are pressed on the shaft. This construction technique helps to reduce engine weight and inertia forces in the camshaft exaggerated by torsional vibration. Solid camshaft lobes may be occasionally induction hardened or even nitrided to withstand high unit loading. Cam lobe profiles are ground to create quieting ramps that minimize valve velocity when seating valves Figure 48. Valve timing (opening and closing in relation to crank position), valve lift, duration, and lobe separation are established by the cam profile. If a camshaft is shared between different power ratings of the same engine model, offset keyways on the cam gear may change valve timing. Cam bushings are used to provide a bearing surface for camshaft journals Figure 49. |
FIGURE 09-47: A typical diesel camshaft. Note the three lobes per cylinder. One is for the injector, and the other two are for the valves. |
FIGURE 09-48: Nomenclature of a camshaft lobe. The closing ramp slows the valve velocity as it reaches the valve seat. |
FIGURE 09-49: In this low-mounted, in-block camshaft, the cam bushings provide a bearing surface for the camshaft journals. |
FIGURE 09-50: Timing marks on the geartrain are used to synchronize the valve operation with the piston position. |
All camshafts require correct synchronization with the engine crankshaft to ensure that the valves open at precisely the correct time in relation to piston position. Timing marks are stamped into the geartrain and are aligned during assembly to establish correct camshaft-to-engine timing Figure 50. A low-mounted camshaft will typically use a single camshaft gear that rotates at half the engine speed to engage the engine’s crank gear.
If a helical-cut gear drives the camshaft, a thrust washer is placed behind the cam gear to prevent the gear from wearing into the cylinder block or head.
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1.1. Overhead Camshafts
LO 9-06: Identify and explain the purpose, construction, and operation of the camshaft. |
Overhead Camshafts
More recently designed overhead camshaft engines often use two camshafts. The Detroit DD13 and DD15 engines use one camshaft for the intake valves, a second camshaft for the exhaust valves, and an additional lobe for each cylinder on the exhaust cam to operate the compression release brake. The rear-mounted geartrain does not use timing marks to synchronize cam-to-crank operation. Instead, it uses special fixtures to lock the camshaft and geartrain in place Figure 51. The first generation of Cummins ISX with unit injectors uses two camshafts as well. One camshaft operates the valves, having lobes for the intake, exhaust, and compression release brake. The other, larger-diameter camshaft contains lobes to actuate the unit injectors Figure 52. The high pressurization of fuel inside the unit injector of the older ISX requires a wider lobe to distribute the injection actuation force. Space on the camshaft would not allow the use of three or four lobes on a single camshaft.
A |
B |
FIGURE 09-52: The wider injector lobe needed on the Cummins ISX requires a separate camshaft for the injector and valves. |
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| FIGURE 09-51: A. The Detroit DD15 engine crankshaft is positioned at TDC and the geartrain is locked in place to synchronize cam and crank operation. B. This wedge tool is placed between the two Detroit DD15 camshafts to properly phase camshaft operation. | |||
| Technicians Tip | ||
| Exhaust cam lobes and exhaust valve train mechanisms have the greatest unit loading pressure, which produces comparatively greater wear in these components. The high force required to open the exhaust valves at the end of the power stroke causes the loading pressure, and several hundred pounds (bar) of pressure remain present in the cylinders after the power stroke in some engines. The wide profile of exhaust cam lobes and rollers helps distribute the mechanical forces over a larger surface area. Injector lobes are even wider to pressurize fuel and prevent camshaft wear. |
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