Powerplants and Drivetrains: Potential Alternatives

Hannu Jääskeläinen, Magdi K. Khair

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Abstract: An important reason for the widespread use of the direct injection diesel engine is its high energy efficiency. The gasoline engine, however, has been making a remarkable progress and its fuel economy has been nearing that of the diesel engine. Further efficiency gains are possible by coupling gasoline and diesel engines with energy efficient drivetrains, such as hybrid electric drives. There are also future drivetrain concepts that eliminate the internal combustion engine altogether, such as electric and hydrogen fuel cell vehicles.

Reciprocating Internal Combustion Engines

Diesel Engines

In most heavy-duty applications, diesel engines are the powerplant of choice because of their high efficiency, reliability and compatibility with the current fueling infrastructure. Diesel engines are overwhelmingly used to power heavy-duty trucks, nonroad machinery in the construction and agricultural sectors, and higher horsepower marine engines. They are also used in stationary applications for power generation, along with natural gas engines.

In light-duty applications in some countries, diesels are also a very popular choice for a variety of reasons including their relatively low operating costs, Figure 1 [3118].

Figure 1. Light-duty powertrain options in new vehicles in several major markets in 2013

(Source: Bosch)

However, the question often arises “Are there viable alternatives to the diesel engine?” Figure 2, adapted from a study by Chrysler Corporation [403], shows that there are few powerplant alternatives that can meet the efficiency of a direct injection diesel engine.

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Figure 2. Energy conversion efficiencies of several powerplants

It should be noted that Figure 2 understates the peak brake thermal efficiency (BTE) of the diesel engine. Light-duty diesel engine peak BTE is typically around 42%. Commercial heavy-duty diesel engines have a peak BTE of about 43% while the US DOE SuperTruck program has shown that greater than 47% BTE is possible with changes to the engine only while adding technologies such as waste heat recover can further increase the BTE above 50% [3083]. Low speed diesel engines have even higher BTE in the range of 55%.

The search for alternatives to the diesel engine can be motivated by a number of factors. Two of the more common motivations are the need for manufacturers to meet government mandated emission standards and a desire by equipment owners to save money.

In most jurisdictions, manufacturers of new engines and equipment must comply with regulations to limit emissions of criteria pollutants. In some cases, fuel economy and/or greenhouse gas emission limits must also be met. As these limits become more challenging to meet, the cost of the additional hardware that must be supplied to the diesel engine can increase significantly. In order to keep equipment cost increases to a minimum, manufacturers may consider other powerplant technologies as viable alternatives to the diesel engine. As an example, in the nonroad equipment sector, there has been an increase the use of gasoline SI engines since the introduction of Tier 4 final emission regulations.

Equipment owners are primarily interested in keeping cost down. In many cases, the total cost of ownership of the equipment purchase being contemplated must be considered. This includes the initial purchase price, the cost of maintenance and repairs and the operating cost associated with fuel and other consumables. An example of this can be found in the use of natural gas engines in some short- and long-haul highway trucking operations where the considerable fuel cost savings can go a long way to offset the added capital cost of the equipment.