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High(mileage)hopes
Inventors throughout history have been laughed at and had their ideas ridiculed. Odds are, each at one point or another has been told: "It can't be done." Today's truck manufacturers are under intense pressure to develop the 10 mpg Class 8 truck. While it's not necessarily a modern marvel like man's first powered flight, the concept is still drawing the same skepticism and doubt.

By Paul Abelson, senior technical editor

Can 10 mpg take flight?

Building the 10 mpg truck will take many paths and involve many disciplines – from engine and drivetrain design to tires and rolling resistance, aerodynamics and vehicle configuration, and even fluids and lubricants.

Each component will play an integral part in achieving the fuel efficiency goal. And each component was the result of a great idea that some inventor or engineering team saw through to the end.

For the 10 mpg truck, it's got to start with the engine.

Engine efficiency refers to how the energy content of fuel is used. There is a fixed amount of energy, measured as British thermal units in a given volume of fuel. For diesel, that ranges from 132,000 to 144,000 Btu per gallon, depending on variations in the crude oil and seasonal blends.

The more the available energy that can be used to propel a truck down the road, the higher the fuel mileage will be. The old rule of thumb was that one-third of the available power was used, one-third heated the block oil and coolant, and one-third went out the exhaust.

Progress has been made over the years, but we're still directing only about 40 percent into propulsion.

When trucks were getting 2.5 to 3.5 mpg, fuel was cheap and rates were regulated. Overall, there was some concern about fuel economy, but not that much. In those days, blowing black smoke was seen as cool. It gave you bragging rights.

Nevertheless, one engine maker printed detailed step-by-step instructions of what not to do because black smoke was the product of incompletely burned fuel, reducing fuel mileage.

Until we began controlling emissions, fuel was squirted into cylinders at injection pressures around 2400 psi. The first steps to creating a more efficient burn, or use, of the diesel included raising injection pressure and redesigning injector nozzles.

Higher injection pressure creates a finer fuel mist. The droplets are smaller, so more surface area of the fuel is exposed in the combustion process. This is important because the surface area is where the fuel vaporizes so it can be burned.

Engine builders soon raised injection pressures above 20,000 psi to get an even finer mist. Pressures continue to increase and now may exceed 30,000. New metals and advances in manufacturing enable injectors to withstand these high pressures, which would have quickly eroded older injectors.

Fuel pressure is but one step in the quest for the 10 mpg truck.

The selection, use and location of metals contribute significantly to improved fuel economy. Stronger, lighter internal engine parts allow engines to change revolutions per minute (rpm) more quickly.

Surface areas where gears, cam lobes and pump shafts and various other components meet can be smaller, reducing friction between those parts and improving response and performance. This leaves more power available to take you down the road, instead of using that power for the engine to run itself.

Engineers are redesigning bearings, a primary source of internal engine friction, as well. While today's engine bearings are still designed and sized to last the design life of the engine, optimizing bearings to take advantage of advances in metals reduces internal friction and lowers engine weight.

Most new engines have gone to overhead cams to reduce weight and internal flex that affects valve timing. Common rail fuel systems are pressurized close to injection pressures. The massive cams, followers and push rods, once used to activate the single injection pulse are no longer needed. That further reduces mass and friction.

Solenoids and electro-mechanical injection precisely time each injection event and accurately dispense the exact quantity of fuel needed for each stroke. Injection pulses occur from three to five times per piston stroke. In the future, many more pulses will be possible.

With such precise injection of exact fuel quantities at pressure levels undreamed of just a few years ago, combustion is almost complete and engines are far quieter. Combustion is now managed, which controls sound and reduces friction.

It takes 2 to 3 percent of the fuel's energy just to pressurize and inject fuel at close to 30,000 psi, but the resulting improvements in economy and engine life are worth it.

More power, less fuel

On the way to the 10 mpg truck, engineers are developing new combustion techniques.

Pre-mix chambers rapidly diffuse injected fuel with a portion of intake air for a more rapid start to combustion and a more even burn in the cylinder. The idea is to inject fuel whenever it's wanted, as much as it is wanted, and burn it instantly.

Turbocharging uses heat from exhaust to spin a turbine wheel. The turbine is connected by a shaft to a fan-like centrifugal compressor that forces additional air into the intake system. That air is further compressed in each cylinder. As a result, its temperature rises above diesel's ignition point so the fuel ignites when sprayed in. That is why diesels are called "compression ignition" engines.

Trade-offs involve size and mass. Without modern technology, a turbocharger would have to be relatively large to handle the volume of air required for a 450- to 600-hp engine. Large turbos take time to spin up, accounting for the turbo lag of pre-'02 engines.

Variable geometry turbochargers maintain high rpm with low volumes of exhaust, but are complicated and costly. Some engine makers switched to twin turbochargers with a small one turning in excess of 21,000 rpm feeding into a larger, lower rpm turbo. One or both may have a waste gate that acts like a safety valve.

Another heat-extraction technology was introduced by Detroit Diesel. Turbo-compounding uses exhaust to spin a turbine that turns a shaft geared to the engine's flywheel.

Part of the energy otherwise wasted out of the exhaust is actually added to the engine's output. When cruising at steady speed, that heat energy is used by the truck, reducing the amount of power that must be created by burning fuel. That increases fuel mileage.

Beyond diesel

Hybrid electric vehicles work in light- and medium-duty trucks, especially in urban use with stop-and-go driving. Reversing their electric motors recovers kinetic energy of the truck's motion, converting it to electricity and storing it for later use.

Hybrids don't do well in steady state, over-the-road driving operations except on hills. But other hybrid concepts offer promise in improving fuel mileage.

Remember, heat recovery improves fuel economy.

One concept is a turbine inside the exhaust pipe, connected to an electric generator. The generator can be inside or outside the exhaust. If inside, it will need high temperature bearings; if outside, it will need a driveshaft seal. This hybrid would continuously recover energy, even in steady state driving.

Another method to capture exhaust heat is to encapsulate the tailpipe in a steam jacket and use it to drive a small steam turbine. Once used, the cooled fluid would return to the steam jacket.

The creativity of the engineers in the trucking industry never ceases to amaze. They have always risen to challenges once thought impossible. Meeting the goal of a 10 mpg truck will be met in the not-too-distant future. And shortly thereafter, driven by demand for ever more efficient trucks, they will exceed it.LL

 

– Edited by Jami Jones, senior editor

Aug/Sept Digital Edition