VINTAGE SMOKE

1927 CUMMINS MODEL P

When 1927 rolled in, the Cummins Engine Company was in it’s eighth year of operation. Founding engineer Clessie Cummins had started in 1919 with a small shop in W.G. Irwin’s garage and a license to build oil engines using the Hvid (pronounced “Veed”) system. That engine, and a disastrous association building them for Sears, Roebuck & Company, almost put Cummins out of business. Clessie engineered his way out of the crisis with innovative new engine designs that began appearing in 1924, and the Cummins Engine Company started on the road to success.

The new era started off slowly but a big boost came in 1925, when Northwest Engineering Company, of Green Bay, Wisconsin, inquired about diesel engines for it’s line of power shovels (we call ‘em excavators today). The Cummins Model F was still fresh off the drawing board and had been well proven as a stationary and marine engine, with one through six-cylinder variants. As part of the agreement with Northwest, Cummins also developed an enlarged variant, the Model N, which followed the same basic design of the F but the bore and stroke was enlarged from 5.5 x 7.5 to 5.75 x 8.5 inches.

The Model F had earned a great reputation with the U.S.

Government, particularly the Bureau of Lighthouses, as a fuel-sipping stationary electrical powerplant but the reputation it acquired in Northwest power shovels was at the opposite end of the spectrum. The laundry list of problems in that applications was extensive. The exposed camshaft and rocker gear did not do well in the extremely dirty environment where power shovels were found and reliability became a big issue.

The dirty environment extended to the fuel as well. Clogged injector nozzles was a common industry problem due to inadequate filtering, poorly refined fuel and carbon buildup. On top of that, the injection system of the F, which worked very well in stationary and marine applications, was illsuited to the operational environment of a power shovel, which varied from a massive load that badly lugged the engine, to running at peak rpms with little load. The injection system didn’t have the flexibility necessary and tended to overfuel during those heavily loaded periods, causing further durability issues.

These problems had solutions and Cummins instituted several right away. Among the first was an air chamber device, technically known as the cup wiper but is better known as the “sneezer.” It was a small chamber in the piston that stored some of the compression pressure and when cylinder pressure dropped after the firing event, that stored pressure blasted fuel droplets off the injector tip so they wouldn’t make deposits. The sneezer carried on in other

Cummins engines into the Model H-lines.

To solve the worst parts of the valvetrain wear issue, the Models P and W debuted. They shared the bore and stroke dimensions of the Models F and N, as well as the per-cylinder power outputs, but they featured an enclosed, one-piece crankcase that housed the camshaft and lifters. The rocker gear was still exposed. The P and W engines came only as four-cylinder inlines, versus the multi-section blocks of the F and N that could be built with one to six cylinders. Cummins also introduced a new injection system for the P and W with better metering capability, which is detailed farther along.

By the time most of the upgrades had happened, both sides were disillusioned and disappointed with each other. Northwest and Cummins didn’t renew their contract and parted ways in 1927. Clessie was later said to have learned a lot about applications from the experience and the company spent more time matching engines to environments rather than simply making sales. These lessons bore full fruit when the fully enclosed Model U debuted in 1928, along with the legendary Cummins single disc injection pump.

The four-cylinder Model P was rated at between 50 and 60 horsepower at 600-700 rpm, producing about 420 lbs-ft at those speeds. According to Cummins historians, only 24 model P engines were built. About half of those went to Northwest before the contract expired and the rest were sold into stationary and marine applications. Cummins historians recently made a count in the records and determined approximately 491 Models F, N, P and W engines built to 1931 before they were completely phased out after being outclassed by the Model U. The legendary Model H (see Diesel World April and May 2019 issues) came just a few years later and eclipsed all of them, soon putting the Cummins Engine Company into full profitability.

Jim Rush restored this 1927 Cummins Model P and it’s the only known survivor worldwide of only 24 Model P engines built. It’s serial number 8184, the 184th Cummins engine built. Cummins started numbering their engines at 8,000 in 1924, the Model F being the first all-cummins design. Number 8184 was sold to the Wentworth Institute of Technology in Boston, Massachusetts, where it operated a 30 KW backup generator for many years. When the powerhouse was torn down, the engine was sold, went thru a couple of owners and ended up with Rush, who has become noted for his diesel restorations. It displaces 712 cubic inches and with it’s particular combination of parts, was rated for 57 hp at 685 rpm, as found on the builder’s plate.

Contrast the 1927 Model P to this 1926 Model F engine, built to Northwest Engineering specifications for use in power shovels. The 66th engine built, it was retained by Cummins as an engineering test unit. You can see the individual cylinders are also part of the upper crankcase and the cam boxes are external on the cylinders. This engine was rated at 50 horsepower at 600 rpm. A couple of special Model F engines were built with aluminum castings to save weight.

The one piece crankcase casting is very evident in this shot of Rush’s Model P. Individual cylinders mount to the block along with cylinder heads. This enclosed the cam and lifters and made for a stronger assembly as well. It was a portly fella, weighing in at a whopping 4,000 pounds. Like most engines of the era, it used cast iron pistons and they mounted 2.25-inch wrist pins. The crankshaft was supported by five main bearings and insert bearings were used.

The external rocker gear was not optimal and Cummins knew it. It wasn’t much of an issue in the engine room of a boat or in a building, but mounted to equipment operated outside in a dusty environment was another matter. The right rocker (notice the roller tip) is the intake and the left is the exhaust. In the center is the injector. One of the big upgrades for the P was a new injection pump that metered internally rather than at the injector. That solved the overfueling issues with the Model F. The injectors were similar, but the P did not have a metering valve in the injector.

The early Model F injection pump (big brass device, lower right) simply delivered fuel at 50 psi to the injectors on a common line. The injector had a metering needle operated by a linkage going back to the governor and controlled the amount of fuel allowed into the cup. The high pressure part of the injection was done by the plunger actuated by the rocker arm, using however much fuel had been allowed into the plunger chamber. The system worked fine in most applications, but not in the power shovel. When digging, the engines were lugged down very hard and to very low rpms, usually with the throttle still in the full power position. The governor would try to speed the engine up by keeping the metering needles full open. At 50 psi, the injection pump would flood the plunger chambers with excessive amounts of fuel and the injector would pump it right into the engine. The engine then would blow excessive smoke, wash down the cylinder walls, generate high exhaust gas temperatures and hurt itself eventually. Not good!

The cup wiper (1), known by most at Cummins as the “sneezer,” was a small air chamber built into the piston crown with a small orifice leading into the combustion chamber. The volume of the air chamber was less that 10 percent of the total volume of the combustion chamber. On the compression stroke, it stored pressure and when the piston started down on the power stroke, that pressure would be released via the small orifice to blast excess fuel from the nozzle tip to prevent deposits from forming. It worked well and had the added benefit of helping to mix the air and fuel a little better during combustion, though that was not it’s primary purpose. The arrow marked “2” indicates the metering needle used on the early Model F injectors.

The updated injection pump (again, the big brass device) had individual plungers for each cylinder, timed to the engine. Fuel delivery was determined by a suction valve for each plunger and controlled by the governor via an eccentric. When the suction valve was closed early, a large amount of fuel would be delivered to the injector and vice versa. You could still potentially overfuel, but not to the level of the Model F. This system would serve Cummins until the single disc pump debuted on the Model U for 1929.