Cummins V-555 “Triple-Nickle” V8 Diesel Engine

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PIX of Cummins V-555

Cummins developed the 555 cubic inch displacement V-555 (Triple Nickel) V8 engine. Like other small V8 Diesel engines, it proved to be a trouble-plagued engine especially in marine service.

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Cummins V-555 “Triple Nickel” Cylinder Block with replaceable “Wet” Cylinder Liners removed.

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Unfortunately, the V-555 suffers from a “weak bottom end”. Reports of “broken cranks”, “spun bearings” and “thrown rods” NOT caused by “Hydrolocking” are far too frequent. This weakness, however is common of most smaller “V” cylinder configuration engines, especially those with cylinder bores less then 5 inches (127mm) and is due to the overall shortness of the crankshaft. With twice as many pistons connected to a V8 crankshaft which is typically only slightly longer then an inline 4-cylinder crankshaft, there just isn’t much room for the connecting rod bearings, the crankshaft main bearings, and the crank webs. Consequently these bearings and webs must be narrower as graphically shown in the picture directly below of a samll V8 crankshaft. Unfortunately, in the case of slipping type bearings such as crankshaft bearings, the narrower the bearings, the more difficult it is maintain an adequate lubricating oil film in the bearings to support the heavy loads of a high compression diesel engine especially under load at higher RPM’s.

Detroit Diesel 8.2 Crankshaft

Even though the connecting rod pins on a V8 crank are the widest journals on the crank, when assembled the V8 crank will have two connecting rods crowded onto each rod journal leaving little room for each individual rod bearing.

V8 Crankshaft with Two Rods assembled to each rod Journal.

In the picture second above and the illustration below, it is easy to see how narrow the rod bearings have to be. Note the two oil feed holes per rod journal (one for each connecting rod. In a fully pressurized lubrication system, the oil is fed to the center of each bearing so that it can form an oil film between the crankshaft journal and the bearing surfaces, as it works its way to the outer edges of the bearing where it squirts out and splatters around the crankcase as the crankshaft turns. Oil tends to exit narrow bearings much more quickly. The wider the bearings, the more oil they can hold longer between the metal surfaces of the crankshaft and bearings, hence wide bearings can carry higher loads. The larger the cylinder bores, the longer the crankshaft and therefore the more room available for wider bearings. V8 engines with cylinder bores larger then 6″ typically have plenty of room on the longer crankshaft for bearings, etc.

Detroit Diesel 8.2 Crankshaft – Illustration.

Note how narrow the five main bearing journals are especially when compared with the 4-cylinder crankshaft below. They will be discussed in more detail a little later.

Another issue with any small V8 crankshaft is the narrow crank webs which are much weaker and consequently much more prone to breakage as shown below.

V8 Crankshaft with broken web near front end of crankshaft (left).

By comparison, the inline 4-cylinder crankshaft below which will have only one rod fitted per journal when assembled will have much wider rod bearings. Note the much wider main bearing journals which will accommodate much wider main bearings. Also note the wider and much stronger crank webs between the bearing journals.

Detroit Diesel 4-53 4-cylinder 2-stroke cycle crankshaft.

The inline 4-cylinder crankshaft shown above has 5 main bearings. The crank webs, and the width and diameter of the main and rod bearings have been optimized to carry the stress and load of a high compression, high output engine. By comparison, the V8 crankshaft has the same number of main bearings for twice as many cylinders, and the V8 main bearing journals are much narrower. You can see how crowded a small V8 crankcase can be in the picture directly below. There just isn’t enough room for rod bearings or main bearings to be wide enough to carry the heavy loads generated by a high compression high output engine.

Notice that the main bearing journals of the V8 crankshafts shown in the illustration and pictures above have been increased in diameter to increase the bearing surface in an effort to compensate for their narrowness. But at some point this becomes counter-productive because the increased diameter increases the slip-bearing surface speed which makes it more difficult for the oil to maintain adequate oil film thickness at higher RPM’s. By comparison, a 4-cylinder crank’s main bearings can be wider so they can more easily maintain oil film thickness and carry the loads, hence the journals can be smaller in diameter to reduce bearing surface speed. This is why inline engines with the wider bearings and stronger crank webs, and main bearings between each cylinder can be air charged (e.g. with a turbocharger) to reliably produce more than twice the horsepower per unit of displacement then a small V8. Air-charging these stronger inline diesel engines can also enable them to run cleaner with fewer emissions.

Engines with longer piston strokes have the advantage of typically producing much higher torque at slower crankshaft speeds (RPMs). Many small V8 engines are designed with a stroke much shorter then most other diesel engines of this displacement. It is just slightly “under-square” with a 108mm bore x 112mm stroke. Besides allowing the engine height to be lower, the shorter stroke also has the advantage of reducing the load on the “bottom end”. Unfortunately, any of these short stroke, nearly-square and over-square engines produce less torque and have to be set-up to run at higher RPM’s to produce their maximum power, which is limited by their increased crankshaft slip-bearing surface speeds. Unfortunately, as bearing surface speed increases, bearing wear increases and so does the risk of bearing failure.

In conclusion

The above comparisons show why smaller V8 engines that lack the space for the wider crankshaft bearings and stronger crank webs are not capable of the higher power outputs of comparable displacement inline engines. These are some of the main reasons why most diesel engine manufacturers have abandoned building small V8 engines and have embraced inline configurations especially the turbocharged 6 cylinder inline with 7 main bearings such as the Cummins B and C series engines. In the case of V8 engines with bores larger than 5 inches (127mm), they are longer and therefore have more room for wider crankshaft bearings and stronger crank webs which means that they can have much stronger “bottom ends” and therefore higher power outputs  per displacement unit then their smaller, shorter, and weaker little brothers.

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During the 1970’s fuel crisis, Detroit Diesel saw the need for a cheap, fuel efficient engine to power medium trucks and school busses. Their answer was a lightweight, slightly under-square (108mm bore x 112mm stroke) 500 cubic inch displacement, four-stroke-cycle V8 diesel engine called the 8.2 Litre. The engine was eventually marinized by a few third-party companies (¿including Johnson & Towers?) and fitted to vessels, but they proved to be quite troublesome as they were prone to head gasket problems due to the lack of a proper ‘deck’ for the the head, and bearing failures due to a weak “bottom end”.

Cummins developed the 555 cubic inch displacement V-555 (Triple Nickel) engine to fill the same market, but it proved to have a weak “bottom end” as well. Caterpillar had came out with the 1100 medium truck engine in the 1960’s which became the 3160 marine engine. It was a larger V8 engine at 636 cubic inches of displacement, but like it’s other V8 rivals, it suffered from weak “bottom end” problems also. In marine use, it also suffered from a design flaw of the camshaft driven gear that sometimes resulted in the gear spinning on the camshaft during a prop-strike. When Caterpillar came out with the 3160’s successor, the 3208, they improved the strength of the “bottom end”, but like the 3160 it still had the camshaft gear flaw.

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