Optimization Potentials of Commercial Combustion Engines

Commercial Powertrains

21. September 2018 | Engineering Service

As the global population continues to grow, the need for increased logistics, infrastructure and agricultural production do also. As a result, improving commercial diesel powertrain efficiency, and reducing its climate and emissions footprint has become a major topic of focus. Using typical commerical applications from the market, we will discuss efficiency enhancing and promising technologies.

Due to the sheer number of units, on-road applications contribute significantly more energy consumption, as well as pollutant emissions, compared to mobile machinery.. Until 2040, commercial on-road vehicles are forecasted to increase energy consumption by 1.4 percent year-over-year. During this time, a majority of the transportation will be sourced by liquid fuels.

Increasing efficiency in basic engine development

Based on several projects and comprehensive studies conducted by FEV & FEV Consulting GmbH regarding powertrain development and optimization, researchers are working on developing current and future requirements for the base engine, which are summarized in Figure 1. Most optimization measures have been available on the market for some time and became established features for reducing fuel consumption. In the near future, we can expect new measures to be launched and others are being intensively researched.

Fig. 1: Improvement potential in fuel consumption compared to additional engine costs

“Downspeeding” and “downsizing” represent effective options for reducing fuel consumption through lowered friction. The goal is to shift the vehicle’s engine operating point close to the optimal Brake Specific Fuel Consumption (BSFC) area in the engine performance map. The average fuel consumption improvement potential is near 2-3 percent. As an alternative approach, FEV investigated the influence of the cylinder swept volume and the number of cylinders, keeping the engine displacement constant. Based on this, different medium-duty (MD) and heavy-duty (HD) six-cylinder inline engines were analyzed and then transferred to four cylinder engines with more than 2 L/cylinder. In addition to weight and package advantages, fuel savings of 3 percent are possible (Figure 2).

Fig. 2: Influence of the cylinder swept volume while retaining the engine displacement. MD commercial vehicle is used as an example


Several other known and established measures can also be applied. For example,cranktrain optimization involves adjusting the crankshaft bearing and journal dimensions, with a potential fuel savings of up to 3 percent. Additionally, advanced coatings can be applied. Reducing the parasitic losses from demand-controlled oil and water pumps, controllable thermostats and switchable piston cooling jets (PCJ), can generate additional improvements of nearly 2 percent. Optimized air path and further increase of the boost pressure using a high efficient turbocharger and two-stage charging are also applied. In regards to thermal management, a variable valve train system with a cam phaser on the outlet sidehas the potential to raise the exhaust temperature, and improve Diesel Particle Filter (DPF) regeneration and warm-up time. Such advantages were verified specially on delivery trucks and off-road machines under, low ambient temperatures and low engine loads.

Waste Heat Recovery Through Organic Rankine Cycle

In addition to the engine-internal measures, the further use of the exhaust gas energy represents an additional potential for the improvement of overall vehicle efficiency and therefore, fuel consumption.
Organic Rankine Cycle (ORC) is the most promising technology for waste heat recovery in commercial vehiclesbecause it has the highest power output in comparison to other technologies. In an ORC,a fluid is condensed isobaric, compressed isentropic, evaporated isobaric and expanded isentropic to convert heat into work. Because of their low boiling points, organic fluids are best suited for medium heat sources, as is the case for exhaust gases from internal combustion engines. One of the major challenges for mobile applications are packaging and the additional cooling demand.
Pre-analysis, based on computational simulation, is a prerequisite for the specific concept decision and integration of such an application. Therefore, FEV developed a holistic investigation and simulation platform, based on established software products.

The vehicle model generates the speed and torque information to define the load points out of the engine performance map. These are generated with a physical and scalable engine model, which simulates the combustion process and the according losses. The ORC model is then fed with information about the exhaust gas temperature and mass flow. In order to not negatively influence the emission reduction, the ORC is placed after the exhaust after treatment system (EATS). Therefore, the simulation of the heat losses between the turbocharger and the EATS are necessary in determining the exhaust gas temperature downstream the EATS.
To evaluate the fuel savings potential for the ORC system which includes, a piston expander, and ethanol, the simulations were performed on a 40-ton on-road HD truck with an 11l, 345kW Diesel engine. The Long Haul cycle, which is an upcoming CO2 certification cycle for HD trucks in the EU, is the basis for the transient simulations. The net power output – expander output reduced by the power needed to drive the ORC pump – was used to determine system performance. The maximum net power output was nearly 14 kW, with an average of 4.46 kW. The ORC had its highest impact on the fuel consumption mainly at high load points during the second half of the cycle, resulting in a simulated fuel consumption reduction of 3.8 percent.

Improving Efficiency Through Hybridization

As we know from passenger car powertrains, the benefit of hybridization or electrification lies in the possible recuperation of mechanical work and the use of regenerative energy sources to charge electric energy storage systems. At the same time, depending on the topology of the powertrain and its application, the corresponding potential can be leveraged by optimizing the configuration of different energy storage systems and drive sources.
For example, Figure 3 presents the results from a study conducted with a 40-ton semi-trailer. In terms of hybrid concepts, the “mild hybrid” concept was examined first. This concept enabled extended start-stop operation with support from the combustion engine in transient operations. “Full hybrid” concepts were also examined, which allowed for maximum recuperation of braking energy and, on top of that, facilitated all-electric driving. This means a correspondingly strong electric machine with roughly 200kW output will be required. The storage size of the electric battery has to be designed based on the desired driving distance. The “Full Hybrid II,” for example, can travel 20 km, or nearly 12.5 miles, on electric power.

Fig. 3: Overview of different hybridization levels regarding efficiency, cost and load capacity

As shown above, a moderate improvement in fuel consumption of 1-3 percent in the Mild or Full-Hybrid I can be achieved, while improvements of up to 18 percent are possible for short-distance city driving. However, in comparison, emissions disadvantages in the “Full Hybrid I”, due to the cooling down of the exhaust system, have been found.. The use of such concepts in long-distance HD trucks is not particularly economically feasible due to the cost and net-weight load capacity disadvantages caused by the high voltage system, the battery package and its overall weight. However, this depends largely on the cost trends in electric systems and the economy of scale.
Taking a serial hybrid concept into account, the energy savings potential is dependent on the load profile/driving distance and primary energy source of about 20 percent (Well-to-Wheel). This compared to base combustion engines was calculated at 100 km/day, power mix.

Summary and Outlook

Based on worldwide scientific and business researches, it is clearly forecasted that internal combustion engines – specifically the diesel engines – will play a major role in the sustainable economic and society development in the future. Therefore, it is mandatory to continue to develop advanced powertrain system technologies and drive concepts, which include internal combustion engines, in parallel with electrification.
Technology measures reducing engine friction and parasitic losses have been discussed and have been already widely introduced in modern powertrains by many manufacturers. Therefore, this paper highlights two technology measures, WHR and electric hybridization, which shows major potential when integrated in the complete powertrains. Figure 4 provides an overview and outlook for the technologies discussed in this paper.

Fig. 4: Current technology readiness for MD and HD powertrain fuel efficiency increase