Over the past two months, we addressed regulatory requirements for fuel economy and greenhouse gas emissions for a number of technologies we can reasonably expect to be incorporated into future truck designs. This issue wraps up the series with a look at technologies for improved combustion efficiency, engine down-speeding and engine parasitic loss reduction.
While these technologies will undoubtedly achieve mandated goals, the first direct impact to fleets will likely be higher prices for new trucks and complex technology requiring 1) additional training for maintenance personnel and 2) new test equipment purchases. It remains to be seen if fuel cost savings will offset additional operating costs.
Improved Combustion Efficiency
One emissions criterion that engine manufacturers struggle to control is oxides of nitrogen (NOx). These form when free nitrogen in the charge air (air is almost 80% nitrogen) reacts with oxygen in the combustion chamber at high temperatures. Most current engines address this issue by using exhaust gas recirculation (EGR) to reduce combustion temperatures. While this is effective in reducing NOx, it also significantly reduces the amount of energy produced by fuel combustion. Future gasoline engines will probably combine gasoline direct injection (GDI) with selective catalytic reduction – currently used on diesel engines – to control NOx while reducing or eliminating EGR. This has the potential to significantly increase combustion efficiency.
Lean burn technologies reduce the amount of fuel burned during the combustion cycle. These have been in use for many years, but lean burn engines tend to operate at higher temperatures (more NOx). SCR technology can potentially allow engines to operate more efficiently (higher combustion temperatures) and burn less fuel (even higher temperatures). The net result? Significantly better gasoline fuel economy. Current engine technologies are capable of withstanding higher temperatures; newer technologies, such as ceramics, may even allow for reduced engine cooling requirements, which will further improve thermal efficiency.
The ultimate goal of engine manufacturers is the introduction of stoichiometric gasoline direct injection.
Stoichiometric or Theoretical Combustion is the ideal combustion process where 100% of the fuel is burned completely. This would result in an exhaust stream containing nothing but carbon dioxide (CO2), water vapor (H2O), and free nitrogen – and the oxygen would have a greater affinity to react with the fuel than it would with the nitrogen . . . thus eliminating oxides of nitrogen. This ultimate lean burn engine will probably never exist due to the variations in operating conditions, but technologies such as series hybrids coupled with advanced computer-controlled fuel injection and charge air management, may allow us to approach this goal.
The greatest power demands occur during initial launch, rapid acceleration, and climbing grades. As a result, most engines generate significantly more power than what is required to maintain cruising speeds on relatively level highways. Advances in engine management systems, coupled with advanced transmissions, may allow engines to operate at significantly reduced RPMs when operating under cruise conditions. This is already done to some degree in over-the-road applications, but if done in advanced transmissions, may allow the use of very low overdrive ratios (.50 or lower) to further reduce engine cruise RPMs. Transmissions may use multi-step fixed overdrive gear ratios, some form of overdrive torque converter, or possibly even continuous variable ratio (CVT) systems. These designs are mainstream in light vehicle applications, but can absorb limited input horsepower.
The challenge from an engine management point of view is to control valve timing, fuel injection rates and timing, and ignition points to ensure efficient combustion at these low RPMs. In general, these engine management technologies are already developed (they may need some fine tuning), so transmission design is the primary challenge. At this point, I suspect this to be primarily an issue of cost as opposed to technology for multi-gear concepts. We already see nine- and 10-speed automatic transmissions coming to market. The development of high horsepower CVTs is still in the future.
Reducing Parasitic Losses
A significant amount of energy developed by an engine is used to overcome internal friction and to operate systems that do not directly contribute to the propulsion of the truck. Work has already been done to reduce these losses, and you can expect additional developments in the near future.
An engine’s coolant pump is typically a belt or gear drive unit that operates continuously. In some situations, this pump circulates more coolant than is necessary since it must be designed to cool the engine in a worst case scenario. The use of an electrically-driven pump that regulates coolant flow to match engine heat loads can contribute to increased efficiency. Coupled with an electrically-driven radiator fan (already in use in many applications), you can achieve even more reduction in parasitic horsepower demands.
The engine oil pump is another 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. At the opposite end of the spectrum, many engines suffer inadequate lubrication at start-up and low idle. This leads to increased friction and engine wear. By substituting an electrically-driven oil pump, we can again match oil flow to engine demands while providing pre-start-up and low idle speed lubrications. 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.
Internal engine losses can also 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 may also 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.
Mechanically driven truck engine / vehicle systems are inherently inefficient. They must be designed to perform satisfactorily at engine idle speed and at maximum engine RPMs. By electrifying these systems, they can be designed to operate at a fixed speed (which is more efficient), and to only operate when needed. For example, an air conditioning compressor must be able to cool the truck’s cab at idle speed (600 RPM) but also be able to function at maximum engine RPM. An electrically drive 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. 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. 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).
The truck engine of the future will be significantly more efficient and clean burning than today’s trucks. It will also be significantly more complex and your maintenance personnel will need to be trained to service these systems. While some technologies addressed over the past several months are still in the lab, many others are just waiting for the regulatory and / or economic drivers needed to justify their deployment. Given the current regulatory demands for improved truck engine fuel economy, I predict that you will see some of these technologies within the next five years or so.
If you would like to discuss this or any other fleet issue with the NTEA, please call 800-441-6832