When I was a small boy I was live baggage on family trips from the Northeast to visit the grandparents a thousand miles away in Indiana. There were no Interstate highways – just the promise of the Pennsylvania Turnpike with its four smooth concrete lanes and many child-pleasing tunnels. The rest of the way was two lanes, small towns, and diners with steamed-up windows. Constantly in the way were highway trucks, only lately powered by weak-but-economical six-cylinder Diesel engines making 160 horsepower. Drivers knew what was next on their routes, so when possible they would dash downhill to speeds over 70mph in hope of then being able to swoop to the top of the next rise without being pushed down into the gearbox by slowpoke old men in hats. That would mean grinding to the top with even more determination to get a good launch onto the next downhill. If there was oncoming traffic, there we would be, imprisoned in second gear behind that truck, then being unable to pass it on the next downhill because to do so would require going 80mph! This was zoom-and-creep travel.
Low Power Diesel Engines
The low power of those Diesels was the result of three basic causes.
First, although experiments with turbocharging had begun shortly after 1900, and despite the fact that thousands of Allied combat aircraft in WW II were powered by turbocharged gasoline engines, it would take the sudden growth of the jet engine industry after the war to make the technology available in truck engine-sized units of the necessary reliability.
Second, Diesel exhaust becomes dark if enough fuel is injected to ‘burn’ more than 80% of the oxygen in the air charge. That further limited performance in comparison with gasoline engines, which burn 98% of their air with no smoke. We’ve all heard the song proclaiming “My exhaust is blowin’ black as coal”; what that meant was that independent operators trying to make ends meet often resorted to “over racking” their fuel injection pumps to increase power. Fuel injectors in those days were mechanical ‘jerk pumps’ in which rotating cam lobes drove tiny fuel pistons whose effective stroke was changed by rotating their sleeves, each of which contained a spiral spill port that terminated injection. All the sleeves were rotated together by a toothed “rack” that engaged gear teeth cut into the sleeves. Travel of the rack was limited at one end by an idle stop, and at the high power end by a stop whose purpose was to prevent smoke. More power was easy to achieve—but at the cost of making smoke.
And third, because Diesel combustion became difficult at higher engine speeds, the upper limit was at about 2250rpm—substantially slower than the take-off rpm of giant wartime gasoline aircraft engines (typically 2700). Horsepower, in the simplest of terms, is combustion pressure times displacement times how often you can perform the cycle—that is, rpm.
Further, the gasoline-burning truck engine wasn’t dead yet. All the school buses that carried me to enforced boredom burned gasoline. Germany, the nation that had contributed so much to Diesel technology between the wars (1919-1939), powered even its largest tanks of WWII with gasoline engines—often as modifications of former airship powerplants. Stranger yet is that Germany’s WWII gasoline-burning aircraft engines all had Diesel-like direct cylinder fuel injection. In the US, the Hall-Scott company had produced vast numbers of attractively-priced and rugged gasoline truck engines—cheap to buy, but thirsty to run. It took an adventurous fleet owner to decide to gamble on the extra cost of fuel injection (reckoned at over 30% of the price of the engine) being worth it in terms of fuel saved. And remember—this was at a time when fuel oil for home heating was just 10 cents a gallon—because per-barrel production cost in the new wonderland of Saudi Arabia was reckoned at a nickel. It all seemed so simple back then—the US versus Soviet Cold War was the only show in town and nobody had heard of terrorists.
It took a long time for Diesel fuel injection to mature, for in the early years the fuel had to be blown into the cylinder with a burst of highly-compressed air (provided by an air compressor built as part of the main engine). Injecting fuel alone (called “solid injection”) took time to perfect because it had to be coordinated with engineered in-cylinder air motion to work properly. Early submarine commanders fretted over the smoke their craft were emitting because, as with F4 Phantoms over Vietnam, it called sometimes unpleasant attention to its source. At present, a nuclear submarine slides beneath the waves and may not see the surface again for months, but Diesel subs were not true submarines. They were submersible surface vessels, operating on the surface at all times when it was safe to do so in order to (1) achieve any significant speed and (2) to charge their batteries to power a strictly limited time of much slower running under water.
STEPS TO DIESEL ENGINE SUCCESS
Another element in the final success of Diesel trucking was the development of tires that could carry heavy loads at even the moderate speeds permitted by the two-lane roads my parents drove in the late 1940s. With tire carcass strength limited by cotton plies (synthetic tire fabrics were developed during the war), it took such a thickness of plies that heat generation in the resulting flexing mass of rubber led to overheating and reversion (really “un-curing”) of tire rubber into a weak, gummy mass. Tire makers began to find some success after 1926 but highways remained primitive until the coming of the Eisenhower National Defense Interstate Highway System, voted into law in 1956. Since that time, a total expenditure estimated at $500 billion (in 2016 dollars) greatly increased the practicality, and cut the cost, of truck transport. Think of the money the railroads could have saved if all their tracked rights-of-way had been paid for from the public treasury!
But, let’s get back to the Diesel and the advent of the turbocharger. Turbine blades for early jet engines were usually forged, but as the hot strengths of fast-improving alloys rose, it became harder and harder to squeeze such resistant materials into the desired shapes. When I was a tech at MIT I used to walk past a door bearing a small plaque announcing that the casting of turbine hot-section parts had been developed in the lab behind the door. When that casting technology made it possible to cast turbocharger turbine rotors in one piece (in modern terms a ‘blisk’—combining BLades and dISK), rather than as the much more expensive disc-and-separate-blades of giant wartime aircraft turbos, the falling price of turbochargers made them practical for automotive use.
Turbocharging made it possible to cram the airflow of a much larger and heavier engine into something that was a practical size and weight for trucking. Every extra pound of structure or drive train is a pound less paying cargo. How much power do you want? You say you need 600hp for crossing those Rocky Mountain passes? Coming right up!
Diesel cylinder blocks were already heavily constructed to handle high compression and combustion loads. Horsepower could now go up quite steeply, making trucks nearly as capable on hills as passenger cars. No more zoom-and-creep!
This happy success story became just another of modern life’s gray frustrations when people began to understand the chemical mechanisms of smog formation. The state of Utah used to offer the condemned a choice of being either hanged or shot—not that different from the choices open to the world’s Diesel emissions engineers: push combustion one way and it becomes more complete, lower in unburned hydrocarbons/particulates, but hotter and higher in nasty nitrogen oxides (NOx). Push it the other way and it becomes cooler, less complete, lower in NOx, and higher in unburned hydrocarbons/particulates. That’s where we are today. Ideas, anyone?
by Kevin Cameron, TDR Writer