It is significant that in 1930 the Packard diesel had a compression ratio of 16:1, whereas in 1931 it has been reduced to 14:1. This was probably done to reduce vibration and the problem of piston-ring sealing.[38] The exhaust products had an unpleasant odor which was particularly objectionable during taxiing. Professor C. Fayette Taylor, writing in the January 1931 issue of _Aviation_, remarked about this fault: "One is inclined to question whether the disagreeable escaping of exhaust gas from the intake ports can be overcome, while still retaining the obvious advantages in weight and simplicity of the single valve." The engine exhaust deposited a black oily film. In fact some airplanes fitted with the Packard diesel engine were painted black, so that soot deposits from the exhaust would not be noticed.[39] Since the passengers' and pilots' compartments were generally located behind the engines, and were not airtight, damage to clothing resulted. This fault could have been eliminated by the use of separate valves for the intake and exhaust systems. It was not possible to start the engine when the temperature dropped much below 32 deg. F unless glow plugs were used. These spark-plug-like devices, which were only used for starting, had resistance windings which glowed continuously when turned on. The additional heat glow plugs provided made starting an easy matter in the coldest weather; however, they complicated the design of an engine noted for its simplicity, and they used so much electricity that only a long flight would allow the generator to fully recharge the battery. H. R. Ricardo, writing in the June 4, 1930, issue of _The Aeroplane_ said: "Referring to the very fine achievement of the Packard Company of America in producing a small radial air-cooled heavy-oil engine, a petrol engine of similar design and with the same margin of safety would weigh less than 1-1/2 lbs. per hp." The important point made is that a gasoline engine designed along the same lines as the Packard diesel would weigh considerably less, but would then suffer from the Packard's reduced structural safety factor. It is significant that as the Packard developed, it became heavier.[40] Like other diesels, the Packard cost more to build than a comparable gasoline engine, because of the type of construction required for the diesel's higher maximum cylinder pressures and the difficulty of machining the fuel injectors. Having fuel injectors, the engine was more sensitive to dirt in the fuel system than a carburetor-equipped gasoline engine.[41] The fuel injectors were "a crude and deficient mechanism" subject to rapid wear, and often these injectors caused smoking exhausts and high fuel consumptions.[42] In the event of battery or starter failure, a comparable gasoline engine could be started by swinging the propeller. Because of the engine's high compression, it would have been impossible to have hand-started a Packard diesel this way.