In the March issue of Fleet AffiliationFleet Affiliation , we addressed the potential impacts of a number of technologies we can reasonably expect to be incorporated into future truck designs, with the primary emphasis on powertrains. This month, we continue our overview of future powertrain technologies.
Improving Thermal Efficiency
A significant portion of energy generated through fuel combustion in an engine is wasted. The primary sources of thermal energy loss are through the engine cooling system and the exhaust system. Considerable work was done in the 1980s and 1990s toward developing ceramic engine components which could operate at significantly higher temperatures, thereby increasing an engine’s thermal efficiency. Unfortunately, higher combustion temperatures also lead to increased levels of mono-nitrogen oxides (NOx) in the exhaust stream. Since reduction of NOx is one of the major exhaust emission reduction criteria, most work in the use of high temperature engine operation was halted. We actually went in the other direction and reduced combustion temperatures through the use of exhaust gas recirculation (EGR).
The reduced combustion temperatures associated with EGR has been effective in reducing NOx emissions, but it has had a negative impact on engine efficiency.
With the continuing development of more and more effective selective catalytic reduction (SCR) systems, we are now looking at the potential to not only eliminate EGR, but to also revisit the use of high temperature engine materials. These two technologies, taken together, offer the potential for significant improvements in engine combustion efficiency.
No matter what we do with internal engine thermal efficiency, we are still losing a significant amount of energy through the exhaust system. Again, engineers are looking at multiple ways to recover at least a portion of this energy. Turbochargers, which operate off of expanding hot exhaust gases, were one of the first steps taken many years ago to recover come of the energy lost through the exhaust system. The typical turbocharger employs a waste gate system to regulate turbo speed and intake manifold boost pressure. The concept of turbo-compounding replaces the waste gate with either a mechanical or an electrical system for utilizing the energy lost through the waste gate. The most practical system appears to be to simply couple an electrical generator to the turbocharger and utilize variable field excitation to maintain a constant load on the turbo. Any energy generated in excess of that needed to provide intake manifold boost would then be converted into electrical energy that can be utilized on board the vehicle in various ways.
Even with the use of a turbocharger, the exhaust gasses leaving the vehicle retain a significant amount of energy. Additional heat may be generated in the catalytic converters associated with the exhaust system. At least two different systems are being developed to capture a portion of this energy. Both of these systems convert the waste heat into electric energy for on-board use. The simplest system uses thermocouple devices to directly convert heat into electricity. A more complex system utilizes liquids with a low gasification temperature, such as ammonia, to drive a small turbine generator system (same concept as electric turbo-compounding) to extract more energy from the exhaust gasses.
The ultimate development of a ceramic engine would be a unit that did not require a liquid cooling system at all. At that point, we might also be looking at the use of an exhaust gas heat exchanger to provide heating for the vehicle’s passenger compartment. This is certainly not a new concept – in fact, it has been in automotive use for more than 80 years. The only question is, would there be enough heat left in the exhaust gasses to accomplish the task, or would we have to supplement the cab heater system with electric heaters?
Reducing Engine Mechanical Losses
The physical operation of the typical reciprocating internal combustion engine requires a significant amount of energy. We have already talked about the energy needed to simply move charged air into the engine and move exhaust gasses out (pumping losses). However, many other aspects of engine operation consume energy. Primary components associated with these losses are:
- Internal friction
- Reciprocating losses
- Valve operation
Two primary sources of internal engine friction losses are 1) piston (ring) to cylinder wall contact, 2) 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. Piston ring friction can also potentially be reduced through the use of exotic materials (such as ceramics) that have a lower coefficient of friction.
The largest solid surface-to-solid surface interface in an engine is the crankshaft. Most of today’s engines use solid (friction) bearings with pressure lubrication. Small engines have been built for many years that use friction-reduction bearing (typically roller style). The assembly of a large engine equipped with friction reduction bearings presents a number of challenges, but we may see such engines in the future. In addition to reducing friction, these bearings would not require as much oil pressure (although they would probably require more volume) for lubrication, so there may potentially be a reduction in energy demands.
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; and 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. 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 RPMs.
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 RPMs with less fuel, so we see a compounded effort when it comes to increasing efficiency.
Four-cycle internal combustion engines require valves to allow charged air into the engine and to allow exhaust gasses to leave the engine. Energy is required to open and close these valves, to overcome the valve spring pressure and to operate the valve actuators. Pushrod engines introduce additional reciprocating mass into the equation in the form of the rods and the rocker arms. Overhead cam engines eliminate most of the reciprocating mass, but you still need some form of drivetrain to operate the cams. Finally, you have 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). 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 has been cost, complexity, and durability. However, we are confident that these issues will be overcome soon.
Regardless of how you actuate a conventional internal combustion engine poppet valve, you still have a number of issues to include valve closing spring resistance and restricted gas flow around the valve. Some work has been done in the area of rotary valves. This concept eliminates the main issues associated with poppet valves but introduces their own set of problems. Again, advanced engineering materials and electronic controls may ultimately overcome the rotary valve issues; we may see them in at least some internal combustion truck engines.
Still More Next Month
This review of future powertrain technologies will conclude in the May issue of Fleet Affiliation with a focus on issues such as engine down-speeding, ultra-high efficiency fuel combustion, and accessory/parasitic engine load reduction.
If you would like to discuss this, or any other fleet issue, please call 800-441-6832