Future environmental regulations will continue to impact truck powertrains. Coming up is Phase II of the Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium and Heavy Duty Engines and Vehicles. This technology advancing program is phasing in over the long term and culminating in standards for Model Year 2027.
“Barring some unforeseen technological breakthrough, internal combustion engines will be with us for some time to come,” says Bob Johnson, who recently retired as director of fleet relations for NTEA, the Association for the Work Truck Industry (www.ntea.com). “In order to meet environmental standards, these engines will have to be both cleaner and more fuel efficient.”
Furthermore, he says many vehicle efficiency technologies being brought online will continue to play a role in the trucks of the future, including:
- Vehicle weight reduction.
- Use of low rolling resistance tires.
- Improved aerodynamics.
- Engine cylinder deactivation.
- Automatic engine start-stop systems (idle management systems).
- Variable valve timing.
- Reduction of engine displacement combined with more efficient turbocharging systems.
- Reduced carbon content alternative fuels (natural gas and propane).
- Systems electrification.
ENERGY CONSUMPTION
One way to improve engine efficiency is to reduce the amount of energy consumed when air moves into an engine and exhaust gases pass out. The energy associated with this basic engine function is referred to as “pumping losses.”
“Turbocharged engines often use some type of heat exchanger system – air-to-air or water-to-air – to cool the intake charge air after it has been compressed by the turbocharger, explains Johnson. “This cooling increases the density of the intake air, which, in turn, helps to improve the engine’s volumetric efficiency. Unfortunately, pushing the air through the heat exchanger takes energy.”
He anticipates a redesigned charge air cooling system that improves the air handling into the engine and new air-cleaner designs, which will allow the charge air to pass through more freely while still removing particulates from the intake air.
EXHAUST GASES
Once the fuel has been burned in an engine, the spent combustion gases need to be removed. The restrictions associated with this process are referred to as “exhaust back pressure.” The first step, Johnson says, is to force the gases from the cylinder through the exhaust valve.
“Look for the increased use of multiple exhaust valves and potentially even totally new valve designs to replace the current poppet valve design in use from almost the entire history of internal combustion engines.”
Removing the exhaust gases from the engine includes the use of mufflers, conventional catalytic converters, diesel particulate filters and selective catalytic reduction (SCR), he adds, and all contribute to exhaust back pressure. “Ongoing research is underway to improve designs for all components, to help reduce back pressure while still allowing the process to function well.”
ENGINE MECHANICAL LOSSES
The physical operation of the typical reciprocating internal combustion engine requires a significant amount of energy. In addition to the energy needed to simply move charged air into the engine and move exhaust gasses out, Johnson says many other aspects of engine operation consume energy. These include:
- Internal friction.
- Reciprocating losses.
- Valve operation.
Two primary sources of internal engine friction losses, he notes, are:
- Piston (ring) to cylinder wall contact.
- Connecting rod to crankshaft journal bearing friction and crankshaft bearing friction.
“A lot of work is being done in the area of improved engine lubricants that will further reduce the friction between these surfaces,” says Johnson. “Piston ring friction can also potentially be reduced through the use of exotic materials, such as ceramics, that have a lower coefficient of friction.”
PISTONS AND RODS
Every time the crankshaft of a reciprocating engine makes a revolution, every piston and rod assembly in the engine must:
- Accelerate from a stop at bottom dead center to a maximum speed at mid upstroke.
- Come to a stop at top dead center.
- Reaccelerate to maximum speed at mid-down stroke before coming to a dead stop again at bottom dead center.
“This process requires a lot of energy,” Johnson points out. “The energy demands can be decreased by first reducing the weight of the reciprocating components – think advanced design and lightweight materials – and by reducing the maximum speed that the components must reach at mid-stroke. This can be accomplished simply by operating the engine at lower rpm.
“The reduction of the engine’s speed also means fewer combustion cycles for any given measurement period, which, in turn, means potentially less fuel burned. All other technologies being incorporated into an engine to increase combustion efficiency/power conversion can potentially allow the engine to operate at lower rpm with less fuel, so there is a compounded effort when it comes to increasing efficiency.”
VALVE OPERATION
Johnson says four-cycle internal combustion engines require valves to allow charged air into the engine and to allow exhaust gases to leave the engine. Energy is required to:
- Open and close these valves.
- Overcome the valve spring pressure.
- Operate the valve actuators.
“Pushrod engines introduce additional reciprocating mass into the equation in the form of the rods and the rocker arms,” he explains. “Overhead cam engines eliminate most of the reciprocating mass, but you still need some form of drivetrain to operate the cams.
“There is also cam-bearing friction and friction between the cam lobes and the tappets. Roller tappets reduce the cam lobe friction significantly but they do not eliminate it.”
Engineers have been looking at various forms of direct valve operation for at least 25 years, with the primary options being hydraulic – engine oil pressure – and electric – solenoid operation, says Johnson. Directly operated valves would reduce internal engine loads and allow for potentially infinite electron engine cam profiles, which would take variable valve timing to the ultimate application.
To date, the major issues with this concept have been cost, complexity and durability, he says. However, he is confident issues will be overcome soon.
PARASITIC LOSSES
The engine oil pump is a device driven at engine speed at all times. Excess oil flow generated when operating at high speeds is typically dumped through a pressure relief valve.
Meanwhile, many engines suffer inadequate lubrication at start-up and low idle, which leads to increased friction and engine wear.
By substituting an electrically-driven oil pump, oil flow can be matched to engine demands while providing pre-start-up and low idle speed lubrications, says Johnson. “The ultimate development would be a dry sump engine – used in some aircraft engines for many years – which would also reduce windage losses created by the engine crankshaft churning the oil in a traditional wet sump oil pan.”
He adds that internal engine losses also can be reduced by using improved lubricant technologies that would further reduce bearing and cylinder wall friction. Developments in materials such as ceramic pistons and/or cylinder wall liners also may contribute to the reduced internal friction.
“We may also see anti-friction (roller) bearings on crankshafts and connecting rods. This is not a new technology, but deployment of high-horsepower engines is problematic at this time.”
SYSTEMS ELECTRIFICATION
“Mechanically driven truck engine/vehicle systems are inherently inefficient,” observes Johnson. “They must be designed to perform satisfactorily at engine idle speed and at maximum engine rpm. By electrifying these systems, they can be designed to operate at a fixed speed – which is more efficient, and to only operate when needed.”
By way of example, he says an air conditioning compressor must be able to cool the truck’s cab at idle speed – 600 rpm – but also must be able to function at maximum engine rpm. An electrically-driven compressor could be optimized to operate at a fixed rpm – which is more efficient – while providing cooling in an engine off condition (idle management technologies).
All proposed electrification load demands on future trucks will require a significantly larger alternator, says Johnson. “Fortunately, electrical systems are inherently more efficient than most mechanical systems, so any way you look at it, there will be some reduction in horsepower demands.
“Belt-driven systems are fundamentally inefficient due to the friction associated with all required drive pulleys, so the next obvious step is to move to a gear-driven alternator. Again, this technology is already used in some applications.”
The ultimate in efficiency will be to incorporate the alternator into the rotating components of the engine, which will eliminate all friction losses associated with driving, he says. “This can easily be accomplished by installing an alternator on the front end of the crankshaft where we currently place the belt drive pulleys, or by incorporating it into the bell housing – think hybrid electric vehicles.”
According to Johnson, the new technologies being added to trucks will impact fleets in several key ways:
- Likely higher acquisition costs for new vehicles and their complex technology.
- Required additional training for maintenance personnel to be able to service new vehicles.
- The need to acquire new test and diagnostic equipment.
