915 GM twostroke diesel

For automotive applications, General Motors offer their 53 and some of their

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3.M.E.P.

Fig. 9.12 Foden ci engine performance curves

800 1000 1200

1400 1600 Rev/min

1800 2000 2200

800 1000 1200

1400 1600 Rev/min

1800 2000 2200

Fig. 9.12 Foden ci engine performance curves

Fig. 9.13 External view of Foden two-stroke ci engine

71 and 92 Series two-stroke diesels, these figures representing, in each instance, the capacity (in3) of one cylinder. The 53 Series are available as three- and four-cylinder in-line, and V6 units, and the 71 Series as three-, four- and six-cylinder in-line, and as six-, eight-, twelve- and sixteen-cylinder V units, while all those with larger cylinders are available in the V layout only. The power outputs of the 53 and 71 Series range from 48 to 597 kW, the larger engines, of course, being for industrial applications. GM claims that the specific power output typical of their heavy duty turbocharged diesels is 5.57 lb/bhp, as compared with 7.78 lb/bhp for an equivalent four-stroke diesel.

Figure 9.14 illustrates the three-cylinder version of the 53 Series, while Fig. 9.15 is a cross-section of the 92 Series with a charge cooler between the banks of cylinders. Although the V-type engines do not necessarily have charge cooling, the latter is nevertheless in most other respects typical of the range.

Mechanical features of special interest include the fact that the crankshaft is a drop forging with journals hardened to a depth such that they can be reground in service, and with fillets that are either hardened or rolled, according to the duty. The camshaft, too, is a drop forging since, under the heavy loading imposed by the unit injection, cast iron would not be adequate. Its cams and journals are, of course, hardened.

To obtain maximum areas of porting for both induction and exhaust, the whole of the periphery of the lower end of the cylinder is taken up by the inlet ports, while the exhaust gases leave through large diameter poppet valves in the cylinder head. These valves are heat-treated nickel steel forgings with hardened stems. With uniflow, instead of loop type, scavenging assisted by a Roots blower and, in some instances a turbocharger too, the incoming air is driven up the cylinder and out past the exhaust valves in the head, sweeping the exhaust before it. This has the advantage of providing cooling

Fig. 9.14 A three-cylinder version of the GM 53 Series two-stroke diesel engines

additional to that afforded by the water jackets. The wet liners are of heat-treated cast iron and are easy to replace.

A recent development is the use of a turbocharger which, for cruising, bypasses its output around the casing of a Roots type blower, the latter being used for scavenging at light load in the low speed range. With this arrangement a smaller blower can be used, saving both weight and parasitic power losses.

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