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Paul von Fange
May 30, 2023
444 views

The Original Two-Engine Scenicruiser

Editor’s Note: This article was extracted from Paul von Fange’s book, Scenicruising: The Greyhound Scenicruiser Story, and was reprinted with his permission.  Before today’s large engines were available, the only solution for gaining more horsepower, was to join two engines together which effectively doubled the horsepower to not only propel the bus up hills better but also to help run the air conditioning unit. This article explains how this system worked and how they could limp home on one engine if necessary, or could they?

Four-cylinder Diesel Engine
(Courtesy of Charles Wotring and used by permission of Greyhound Lines, Inc.)
Four-cylinder Diesel Engine (Courtesy of Charles Wotring and used by permission of Greyhound Lines, Inc.)

As Orville Caesar and Greyhound moved to their deck-and-a-half Scenicruiser in the early 1950s, they had multiple requirements that led to a two-engine solution. First, there needed to be plenty of muscle to move 20 tons of vehicle up the hills and mountains. Second, and in contradiction to the first requirement, fuel economy was extremely important in keeping operational costs down. And last, if breakdowns and the need to send another bus out could be reduced, operating costs would go down further. With the dual coupled engines, a solution emerged that appeared to do it all.

In the early 1950s the existing 225-horsepower six-cylinder 6-71 (six cylinders, each 71 cubic inches) diesel engine did not provide sufficient power or torque for the newly designed PD-4501, and the 8-cylinder version was not to appear until 1960. So, Greyhound and GM came up with a customized solution: twin 150-horsepower 4-cylinder diesel engines side-by-side, connecting together via fluid couplings to a transfer case connected to a single clutch and transmission. The engines were dual 4-71 GM Diesels (name changed to Detroit Diesel in 1965), four cylinders in-line, each at 71 cubic inches for a total of 4.7 liters or 284 cubic inches displacement and, of course, a grand total of 568 cubic inches. They were fed by a 180-gallon diesel fuel tank.

Milo Dean’s Patent “Control for
Two Engine Vehicles”
(Courtesy of the United States Patent and Trademark Office)
Milo Dean’s Patent “Control for Two Engine Vehicles” (Courtesy of the United States Patent and Trademark Office)

Undoubtedly Milo Dean, chief Greyhound engineer, was the key driver in this joint venture. Leveraging his engine-coupling work with GX-1 and his patent 2,689,013, “Control for Two Engine Vehicles,” he and GM engineers produced a hydraulic transfer assembly that yoked the two diesels together.

Today many feel that the design, while certainly proved in practice, was not a successful concept. However, it was an elegant and complex design, reality notwithstanding!

The PD-4501 Dual Four-cylinder Engines
(Courtesy of Charles Wotring, used by permission of Greyhound Lines, Inc.)
The PD-4501 Dual Four-cylinder Engines (Courtesy of Charles Wotring, used by permission of Greyhound Lines, Inc.)

This engineering approach was successful enough in the GX-1, which coupled two air-cooled gasoline engines together, that it became the model for the water-cooled, diesel version that powered the Scenicruiser. In fact, the very words used in stories describing the capabilities and design features of each are amazingly parallel: 

GX-1 “Under normal operating conditions, one engine will propel the bus while the second engine will operate the accessories, including the air-conditioning system, the generators, the air spring compressor and the power hydraulic units. On grades or at other times when extra power is needed, the second engine cuts in automatically to maintain a constant speed without the necessity of shifting gears.” (“Presenting the Greyhound Highway Traveler,” The Greyhound Corporation, p. 4.)PD-4501 “These [engines] drive the coach through two fluid couplings connected with a multiple speed transmission, and operate the generator and air compressors. A power take-off from the right engine operates the air-conditioning system.” [The fluid coupling provided the power of both engines.] (“New Scenicruiser Makes Debut,” Greyhound News, August 1954, p. 2.)

The two engines were mounted at an obtuse (a wide 140-degree V) angle to lower the overall height of the power plant, and each had its own hydraulic coupling to a common gear train.

From the author’s collection and used by permission of General Motors)
From the author’s collection and used by permission of General Motors)
(From the author’s collection and used by permission of General Motors)
(From the author’s collection and used by permission of General Motors)

The engines could be stopped in an emergency without opening the engine compartment doors. An emergency stop switch was accessible through the vents of the left-side engine door. 

The power plant was accessed through dual rear doors hinged at both corner posts to swing out from the middle. The engines as a unit could be pulled out using a custom dolly with leveling jacks, allowing the component that caused the majority of downtime to be swapped quickly. All fluids were either self-contained or, like the air, Freon, and electrical connections, had quick-disconnect couplings, eliminating the need for bleeding or priming.

The Left Engine Close-Up
(Courtesy of Charles Wotring, used by permission of Greyhound Lines, Inc.)
The Left Engine Close-Up (Courtesy of Charles Wotring, used by permission of Greyhound Lines, Inc.)

Spare engines were placed in strategic locations along routes to facilitate the rapid restoration of service should an engine fail. At least that was the plan. But in the early years, this did not happen as anticipated because “A factor which contributed to these delays was the shortage of vital power-plant replacement parts.” According to Greyhound statistics, almost 88% of mechanical failures were due to the power plant. The engine design and location of spares was intended to keep the fleet rolling and reduce the many extra but empty buses on standby, dispatched when a bus broke down on the road. Thus, the Scenicruisers represented “a new concept in bus operation and maintenance.”

Details on the Disconnects and Dolly
(From the author’s collection and used by permission of Greyhound Lines, Inc.)
Details on the Disconnects and Dolly (From the author’s collection and used by permission of Greyhound Lines, Inc.)

The cooling system capacities for the right and left engines were 58 and 52 quarts respectively – that’s a total of almost 53 quarts of antifreeze alone for the northern tier of the country! Each engine’s crankcase held 16 quarts of oil. The transmission had a whopping 38 quarts of fluid, which probably included the hydraulic transfer cases. The power steering held six quarts, and the drive-axle capacity was 13 quarts.

Dual-engine Oil Fillers and Dipsticks
(From the author’s collection and used by permission of General Motors)
Dual-engine Oil Fillers and Dipsticks (From the author’s collection and used by permission of General Motors)
Dual-engine Power Train
(Courtesy of Charles Wotring and used by permission of General Motors)
Dual-engine Power Train (Courtesy of Charles Wotring and used by permission of General Motors)

Drive with One Engine?

The other key feature Greyhound intended to deliver in this coupled-engine design was the ability to run the bus on either engine should one fail. “In the event either engine should fail, manual operation of the fluid coupling by the driver will permit the other engine to propel the bus and run the accessories.”  Depending on whether it was the right-engine or the left-engine that faltered, the procedures to run on each engine alone differed. Internally, air pressure was used to remove fluid from one of the couplings and existing hydraulic fluid paths were altered.

While complex, the primary effort was done with a lever accessible inside the left rear engine door. This lever was normally sealed at the BOTH ENGINES position and the driver would have to break the seal and move it to either the right or left engine position. 

Engine Control Valve and Rear Switch Panel
(Courtesy of Ken Anderson and used by permission of General Motors)
Engine Control Valve and Rear Switch Panel (Courtesy of Ken Anderson and used by permission of General Motors)

Specifically, the operating manual provided these details:

If the RIGHT Engine Failed

A/C off since right engine operates it

Open engine rear and left side doors

Turn right-hand engine switch OFF

Break seal on engine control valve and turn lever to LEFT ENGINE ON

Turn left engine control switch to REAR RUN

Start left-hand engine with left hand starter switch

Close doors and proceed

If the LEFT Engine Failed

Open engine rear and left side doors

Turn left-hand engine switch OFF

Break seal on engine control valve and turn lever to RIGHT ENGINE ON

Make sure right engine starter switch and control switch are in NORMAL RUN position

Close doors, start engine in normal manner, and proceed.

When running on one engine, drivers were instructed to do so only in emergencies, such as getting passengers to the next town for safety or to drive where necessary repairs could be made. 

But there was more. If a driver was unlucky enough to lose the left engine, the power steering was gone, and manual steering was the only option. Also, added later to the maintenance manual (but not mentioned in the operating manual), the exhaust tailpipe on the dead engine had to be sealed in order to prevent the Roots blower on the live engine from pulling dust and dirt through the open exhaust valves in the dead engine. This sealing was typically done with tape or rags!

The common notion that a Scenicruiser could not run on only one of its two engines comes from Al Meier in his 1974 article on the coach’s 20th anniversary: “and contrary to what is often believed, the Scenicruiser could not be operated on a single motor.” In fact, Meier claimed that Greyhound only coupled the engines together for the horsepower and not to run the bus on a “spare” engine! Bob Redden repeated the notion: “and they could not run on one engine, contrary to popular belief.”

However, as seen earlier, Greyhound both designed and claimed that the 4501 could run with one engine. Statements like this are common: “Furthermore, the fact that the Scenicruiser can be driven into a maintenance center on a single engine will serve to reduce road failure…” “The engines are mounted in the rear as a unit, yet each is capable of propelling the bus in the event the other should fail. Thus, the Scenicruiser has what amounts to a spare engine.” “Should either engine become inoperative, [the driver] can manipulate a special lever in the engine compartment to de-activate the faulty engine and permit the use of the other in bringing in the bus.” “In case of emergency either engine can propel the bus.” “If a driver ‘loses’ an engine on the road, he just flips a lever that pumps the oil out of its coupling into a storage reservoir. Then he can come home on the other engine.”

One retired driver who drove the original Scenicruiser, remembered well driving on one engine from Beloit to Chicago at 40 MPH (Ken Anderson, April 17, 2014) Other drivers have similar stories. There wasn’t any technical reason why one engine couldn’t move the 20-ton loaded Scenicruiser at (eventually!) 40 MPH on a level road, and drivers claim to have done it. However, it wouldn’t have been fast, and it wouldn’t have been fun! But, as will be seen in a subsequent article, “Repeated and Extraordinary Mechanical Failures,” This specific capability was the most significant issue encountered with the early Scenicruisers. And, because of what happened, Meier’s comments will turn out to be “virtually” true!

Article written by Paul von Fange
Paul von Fange unexpectedly discovered the Greyhound Scenicruiser in 2009 and has been on a quest for its story ever since.He has ridden thousands of miles on these buses, attending the 2010 Scenicruise in Amarillo, Texas, the 2011 Scenicruise in Grand Canyon Caves, Arizona, the 2011 Gathering of Buses in Hibbing, Minnesota, the 2013 Ghosts of Highway 61 in Blytheville, Arkansas and the 2017 Busboy Rally in Evansville, Indiana. He served as a volunteer at the Minnesota Transportation Museum, working to restore Scenicruiser PD-4501-739.His Scenicrusing videos can be found on YouTube by searching for “paulmontry.” Any additional evidence for the Scenicruiser history can be sent to him at paul@scenicruising.com. His books and more links are available at www.Scenicruising.com.
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