Opposed-Piston Two Stroke Engine as a High Power Density Solution in Heavy Duty Applications: An Experimental Evaluation With Diesel and Ethanol

Author

• Ankur Bhatt, Clemson UniversityFollow

Date of Award

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Automotive Engineering

Committee Chair/Advisor

Dr. Benjamin Lawler

Committee Member

Dr. Brian Gainey

Committee Member

Dr. Robert Prucka

Committee Member

Dr. Harsh Sapra

Abstract

With an increasing demand to reduce greenhouse gas (GHG) emissions, the conventional powertrain technology is at a saturation point. There is reinvigorated research interest in alternative powertrain designs that offer potentially higher efficiencies and lower engine-out emissions compared to conventional powertrains. The opposed piston two-stroke (OP2S) engine is one such alternative engine architecture that has demonstrated a reduction in engine-out emissions and increased efficiency compared to conventional four-stroke diesel engines. Like any two-stroke engine, the scavenging process and the composition of the internal residuals are predominantly governed by the pressure differential between the intake and the exhaust ports. Without dedicated pumping strokes, the two-stroke engine architecture requires external devices to force burned gases out of the cylinder and fresh charge into the cylinder. Thus, the selection and design of the airpath is critical in realizing the benefits of the OP2S engine architecture.

This work investigates two OP2S engines with different airpath configurations and compares the two layouts. Both belong to the same family of engines with different displacements but with the same stroke to bore ratio. The experimental results show that the airpath equipped with a supercharger and a turbocharger (turbo-super layout) had a better scavenging control compared to the airpath with an electrically assisted turbocharger (EAT). The turbo-super layout for the three-cylinder also has the added flexibility to split the air induction between the supercharger and the variable geometry turbocharger (VGT) which regulates its pumping work, unlike the electrified airpath which solely relies on the electrically assisted turbocharger. A workaround to decouple airflow and port pressures for the two-cylinder is suggested and validated in this work. While lowering the scavenging efficiency to under-scavenged conditions reduces pumping work, it also causes an increase in heat transfer losses and reduced thermal efficiency. On the contrary, over-scavenged operation reduces the brake efficiency due to an increase in the external pumping work. This dissertation also showcases experimental results related to controlling the amount of trapped hot residuals (from 20% - 70%) in the OP2S architecture to enable autoignition of low cetane fuels like ethanol across the entire engine operating regime. In doing so, compression ignition of ethanol over the full operable load range was achieved. These experiments also demonstrated that certain trace species in the hot residuals, such as nitric oxide (NO), can have an impact on the ethanol’s reactivity and its combustion stability, introducing a potential "runaway" effect on combustion phasing in advanced combustion modes at high loads. To better understand the effects of NO on ethanol’s reactivity in a controlled setting, a four-stroke engine was used to study this effect in depth in this work. The experimental results were supplemented with the findings from a constant volume reactor modeled in CHEMKIN.

This dissertation concludes by developing a Class-4 and a Class-8 truck model in GT-Drive using the OP2S engine maps and an optimized gear shift strategy custom-developed for these maps. The vehicle model integrated with the OP2S engine outperformed a comparable powertrain with a conventional four-stroke engine. Different hybrid powertrain configurations were then modeled in GT-Drive, each with its own unique rule-based supervisory control strategy. The benefits of hybridization at the component and the aggregate level were evaluated through pseudo-HIL tests over regulatory drive cycles and HIL engine transient load steps. This evaluation can be used as archival knowledge to determine which hybrid layout suits the OP2S engine better based on the application.

Recommended Citation

Bhatt, Ankur, "Opposed-Piston Two Stroke Engine as a High Power Density Solution in Heavy Duty Applications: An Experimental Evaluation With Diesel and Ethanol" (2025). All Dissertations. 4189.
https://open.clemson.edu/all_dissertations/4189

Author ORCID Identifier

https://orcid.org/0009-0002-8486-1766