Many off-highway applications demand a relatively high low-end torque compared to on-highway applications. Because of this, applying downsizing strategies for non-road applications while meeting the operational requirements of the machine can be challenging. Still, downsizing can result in efficiency improvements as well as reduced production costs and packaging benefits.

Certain applications in the off-highway market are suited for engine downsizing. One example is a midsize tractor (Figure 1), which can be used for heavy field work that requires the engine to operate at medium to high speeds with a high load factor, and ancillary farm activities that are transient and for which the engine is concentrated toward low to medium engine speeds.

Various engine and aftertreatment configurations can be applied to meet Tier 4 Final emissions standards. The most common component used in all configurations is the selective catalytic reduction (SCR) catalyst for nitrogen oxide (NOx) reduction. Most applications do not apply a diesel particulate filter (DPF) and rely primarily on low particulate matter (PM) combustion systems to meet Tier 4 Final PM emission levels.

FEV researchers conducted a study to understand the challenges of downsizing a diesel engine from a six-cylinder 7.5-L to a four-cylinder 5.0-L while maintaining performance. They pursued four technology paths. First, an electrically assisted compressor was used in series with a single-stage mono scroll turbine. The single-stage turbocharger was optimized for the peak power operating point, and the E-booster was used to achieve the low-end torque targets. Three different charge air cooler (CAC) placements were analyzed. The best brake specific fuel consumption (BSFC) was obtained with use of two CACs: one placed downstream of the E-booster and the other downstream of the single-stage compressor.

In a second technology path, an asymmetric twin scroll turbocharger was applied to the downsized engine. This application was useful in increasing the low-end torque beyond that possible with a mono scroll turbine, but it did not meet baseline targets.

In a third path, a two-stage turbocharger was mated to the downsized engine (see Figure 2). In this configuration, two air system layouts were analyzed. In the first layout, both turbochargers were sized to be active across the complete load range. In the second layout, the high-pressure turbocharger was sized to achieve the low-end targets, while the low-pressure turbocharger was sized for optimal operation at the higher speed points. This two-stage sequential turbocharging layout showed better BSFC from the peak torque to peak power operating speeds when compared to a conventional single-stage configuration.

In the final technology path, variable valve timing and actuation were used to increase the turbine power at lower engine speeds. To achieve the low-end torque targets while minimizing the engine pumping mean effective pressure (PMEP), the exhaust timing and lift, along with intake valve timing, had to be optimized at each engine speed. The final configuration consisted of a VVA setup on the exhaust valves and a VVT setup for the intake valves to apply optimum valve timings and durations from 600 rpm to 1400 rpm.

When comparing the best-performing case for each technology that achieved the torque, lambda, and EGR targets, two-stage sequential turbocharging was found to have the best BSFC across the complete full load curve, along with the lowest heat rejection to the cooling system. The E-booster configuration performed similarly. For the two-stage sequential turbocharging layout, the BSFC improvement at the peak power point was approximately 6.8%, while the heat rejection reduced by approximately 5.9%. The two-stage sequential turbocharging layout also improved transient performance and delivered better cycle averaged fuel consumption when applied to the two midsize tractor operating profiles.

Results showed that the most effective technology to downsize a turbocharged diesel engine is to add an additional compressor stage, whether in the form of a conventional turbocharger or an electrically assisted compressor. Further investigations are needed to evaluate options to manage peak cylinder pressure requirements, perform cost and packaging studies, evaluate overall system control complexity, and examine the emissions and aftertreatment system sizing based on peak BMEP levels. Ultimately, the transient capability of the selected air path system must be considered and evaluated relative to the required engine performance.

This article is based on SAE technical paper 2016-01-8058 authored by Mufaddel Dahodwala, Satyum Joshi, Hari Krishnamoorthy, Erik W. Koehler, and Michael Franke of FEV NA. The paper will be presented at the 2016 SAE Commercial Vehicle Engineering Congress.