Medium- and heavy-duty vehicles use 26% of all U.S. transportation liquid fuels. That’s expected to increase at least until 2035, according to the Department of Energy (DOE) and the Energy Information Administration (EIA). Also of concern are the greenhouse gases emitted by these trucks, begging the question: what can we do about it?
In 2008, I was asked, along with 18 other committee members from academia and industry, to participate in a study on how to improve fuel economy in medium- and heavy-duty vehicles. The recommendations that resulted were published in the 2010 National Research Council (NRC) report: Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. And, it’s from these recommendations that the government drafted the most recent mandates on fuel efficiency and CO2 emissions for medium- and heavy-duty trucks and buses.
As part of the NRC committee, I chaired the group tasked with writing Chapter 4, “Powertrain Technologies for Reducing Load-Specific Fuel Consumption”. This included diesel engines, gasoline engines, transmission and driveline technologies, and hybrid powertrains. I previously participated in the NRC review of the 21st Century Truck Partnership projects, which has DOE goals for brake thermal efficiency of 50% and 55%.
The diesel engine technologies identified for potential fuel consumption reduction were:
- Turbocharging, including turbocompounding
- Variable valve actuation
- Advanced exhaust gas recirculation
- Electric-driven accessories
- Engine friction reduction
- Alternative combustion cycles
- Improved DPF and SCR systems
- Thermal insulation of ports and manifolds
- Improved work extraction from the combustion process
- Electronic controller calibration management
- Bottoming cycles
A roadmap was included in the report for improving diesel engine thermal efficiency from 42% in 2008 to 49.1% by 2016 and 52.9% by 2019 using some of the above technologies. A similar approach was taken for gasoline engines and it was concluded that gasoline engines could reduce their fuel consumption by up to 24%. Even with this improvement, diesel engines will continue to have lower fuel consumption than gasoline engines by 6% to 24%. In addition, it was observed that, due to the cost of diesel engine aftertreatment systems, diesel engine penetration in medium-duty trucks decreased from 76% in 2004 to 58% in 2008.
As part of this research, we also found that automatic transmissions can reduce fuel consumption by decreasing driver variability and hybrids, both electric and hydraulic, can reduce fuel consumption by 5% to 50% depending on the application and duty cycle.
Overall, the report concluded that three technologies could best contribute to fuel consumption reduction from medium- and heavy-duty vehicles. These technologies are advanced diesel engines, hybridization and aerodynamics.
While reducing fuel consumption in medium- and heavy-duty vehicles is a complex problem with no one “silver bullet” solution, development of new technologies—like the Achates Power opposed-piston, two-stroke diesel engine—will go a long way in helping manufacturers and fleet owners meet the new fuel efficiency and emissions standards for 2014 and beyond (especially when those technologies can be produced for less!).
For the conventional IC engines downscaling the motor helps a lot because the motor is operated closer to its the peak cylinder pressure mode with much better brake thermal efficiency.
http://ratherlightvehiclecompany.com/fueleconomy.html claims that a typical gasoline engine may double the fuel efficiency if the engine is suddenly cut in four (25% at 100% load while 13% at 25% load), see efficiency vs. engine angular velocity and engine load diagram. While checking the same type of diagram for different engines, and comparing real life data, although the gain seems to be a exaggerating, the trend is still the same.
Since the operator does not want to compromise on the available max power one way to resolve this is to decouple, let say, half of the cylinders.
Is variable displacement an option for the Achates Power OP engine? The classic cylinder deactivation achieved by keeping the intake and exhaust valves closed is developed for motors with valves so port scavenging engines may need a different approach. Or you do not consider it because the partial load efficiency is so close to the wide open throttle load peak efficiency that it is not worth the added complexity? Do you have any estimation how much improvement can be achieved?
Thanks,
Sventin
Sventin:
Variable displacement gasoline four-stroke engines are in production today in a number of automotive applications, including larger-sized pick-up trucks. For an unloaded pick-up truck cruising down a flat highway, the cylinder de-activation allows the engine to run on higher specific power which, as you mentioned, improves the fuel economy. The gain in efficiency is reflected by the reduction in pumping losses due to opening the throttle of the gasoline.
In our two-stroke engine, we are managing the pumping losses via the scavenging process. The uniflow scavenging process with intake and exhaust ports enables us to fill the cylinder at part-load conditions with just the right amount of air for combustion without throttling. This filling is controlled by varying the pressure ratio across our supercharger using a recirculation valve. In this way, we have a variable displacement engine without the complex valvetrain mechanism of the gasoline engine.
Gerhard Regner
Director, Applications Engineering
Achates Power
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