By Michael Lamm: From 1934 through 1938, General Motors built three small, rear-engined, experimental economy cars. This was in response to similar projects at Ford and Chrysler. All three manufacturers knew what the other two were doing, and while no American automaker felt serious about producing such cars, no one wanted to be left behind.
GM’s effort began in 1929 when Charles F. Kettering code-named the project “Research Light Car Body.” More than a body, though, it soon became an entire car, including an all-new and highly unusual engine.
- In the lead photo Charles F. Kettering, head of GM Research Labs, inspects Martia I during a shakedown run near Gaylord, Mich., in 1935. A radical 2-stroke X-4 engine stood at rear of car, and it’s doubtful that GM Art & Colour had anything to do with the body design. This was strictly an experimental engineering project.
Kettering set up a formal Automotive Design Department in 1933 and put engineer O.E. (Olle) Schjolin in charge of it. Schjolin was assisted by Carl A. Lindblom and a small, dedicated staff. Together, they had the first car, which Schjolin named Martia I, running and ready for testing in February 1934. His team would soon build a second version, Martia II, and then a final one, named AD-800. All three used fully unitized bodies – fairly unusual for that day – and Dubonnet suspension units at all four corners. But what really set these vehicles apart were their engines – as strange as they were innovative.
- Both Martias had twin radiators up front. Marita I also had a third radiator above the engine.
- Both Martias and the AD-800 seated four and delivered up to 45 mpg.
Not only were they X-4s, placed in the rear of each vehicle, but they were two-stroke and used a Roots-type supercharger. Each arm of the X-4 contained two parallel cylinders and pistons, a U-shaped combustion chamber and two separate connecting rods that attached to a single throw of the crankshaft. Technically, then, these were eight-cylinder engines, but the twin pistons were offset by 15 degrees. One piston took care of intake and the other performed exhaust functions. Schjolin and Lindblom found that offsetting the pistons gave a smoother idle and got rid of the typical two-stroke’s poppety-popping on deceleration. Also, the 15-degree phasing in the side-by-side bores allowed the exhaust to open far ahead of the intake, yet both ports closed at the same time. Thus at top dead center, compression was held for a longer time.
- Two-stroke engines used a Roots-type supercharger to extract the exhaust, not to boost intake volume. Twin pistons in each arm of the X made this an 8-cylinder engine. Left piston was for intake, right for exhaust, and they were phased 15 degrees. The rotating and reciprocating parts can be viewed (above and below) at two different crankshaft positions.
The supercharger acted not as a blower but as a scavenger pump so that, rather than forcing the fuel mixture into each working cylinder, it removed spent gases after combustion had taken place. And because two-strokes don’t burn all the fuel mixture totally, part of the exhaust stream was rerouted around the supercharger and back into the intake. That, according to Schjolin, made these two-strokes 15-20% more efficient than comparable-displacement four-strokes. In these three experimental cars, which weighed 1844-2100 pounds, Schjolin’s team recorded up to 45 mpg in typical driving. Oil consumption was roughly 3000 miles per quart, and oil entered each engine through an external reservoir rather than being mixed with the gasoline.
The two Martia engines displaced 130 cubic inches, the AD-800 160. One of the great drawbacks of these powerplants was their exhaust odor at light loads. George Hallett, one of the engineers I talked to about this project back in 1971, said the exhaust smelled “…like a combination of tear gas and skunk.” Another engineer, Marion Fast, told me, “…these were the stinkiest engines ever built – absolutely impossible from an emissions standpoint, even in those days.” But by force-scavenging both the 130 and the 160 with the same-sized pump (supercharger), the team found that the larger engine had less blow-through and a less obnoxious exhaust smell.
- Some of the exhaust stream was rerouted around the blower and back into the intake. Unlike the previous cutaway, this schematic shows exhaust gases being drawn from the crankcase.
All three engines were water-cooled. The first car had triple radiators, two in front and one above the engine. The Martia II and AD-800 used just the forward radiators. And the AD-800’s engine stood at 90 degrees to the rear axle, while the Martia I and II placed their crankshaft in line with the axle. All three cars used three-speed transaxles specifically engineered for these cars.
The light-car research experiment must have been quite expensive, even by General Motors standards. The project ended when Schjolin and Lindblom left GM to go to Volvo in 1938. Lindblom told me years later that the postwar Volvo PV-444 looked like a 1941 Ford because he and Schjolin wanted to cash in on Ford’s popularity.
Meanwhile, during WW-II, another team of engineers installed variants of the two-stroke X-4 in otherwise stock Chevrolets and Oldsmobiles. These engines performed well, with very little maintenance, as did a similar GM X-4 installed in a small Cessna airplane. Copyright © 2015 Michael Lamm.
- Experimental 2-stroke engines powered not only the three prewar compact cars but also an Oldsmobile and a Chevrolet during WW-II and later a Cessna airplane. All engines worked well, but the main objection was their exhaust odor, which one engineer described as a combination of tear gas and skunk.