Thermal Systems & Emissions

Cutting Diesel NO_x with Hybrid Air Preheating and Urea SCR

A hybrid air-preheating and urea selective catalytic reduction system achieving up to 55 percent NOx reduction and 27 percent noise reduction on a single-cylinder diesel engine, with improved brake thermal efficiency.

By Musa Ibne Mannan · Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas

The trade-off this project attacks

Diesel engines present a well-known and frustrating coupling. The high combustion temperatures that make them thermally efficient are exactly the conditions that drive nitrogen oxide formation. Suppress the temperature to control NO_x and efficiency falls; chase efficiency and emissions rise. Most practical solutions therefore operate downstream of combustion rather than trying to break the coupling inside the cylinder.

Selective catalytic reduction using urea is the established after-treatment route: urea decomposes to ammonia, which reacts with NO_x over a catalyst to yield nitrogen and water. But SCR is temperature-sensitive. Below its effective window the conversion efficiency collapses, which is why cold-start and low-load operation remain the hard cases. This project tested whether pairing SCR with intake air preheating could improve the outcome across the operating range.

System design

The work combined two interventions on a single-cylinder diesel test engine. Air preheating raises the intake charge temperature, which affects both the combustion process and the thermal state of the exhaust stream entering the catalyst. Urea SCR then performs the chemical reduction of the remaining NO_x. The point of the hybrid arrangement is that the preheating stage helps bring the after-treatment system into its effective temperature window rather than leaving it dependent on engine load alone.

Testing measured NO_x concentration, noise output, brake thermal efficiency and specific fuel consumption across engine operating conditions, so the emissions result could be assessed against its cost in performance rather than in isolation.

Results

The headline figure is the 55 percent NO_x reduction, but the result I find more notable is that brake thermal efficiency improved and specific fuel consumption fell at the same time. Emissions control systems normally extract a fuel penalty. Here the preheating contribution to combustion quality appears to have offset it, which means the system is not simply trading one performance metric for another.

The 27 percent noise reduction was not the design target but follows from the effect of intake air temperature on the combustion pressure rise rate — a gentler pressure gradient produces less combustion noise. It is a useful secondary benefit for any application where operator exposure matters.

Origin

This work grew out of a team project I led at Rajshahi University of Engineering and Technology on diesel engine inlet and outlet modification for NO_x control, and was developed through to publication in Heliyon in 2025.

Publication

M. S. H. K. Tushar, M. I. Mannan, A. Azmain. “Exhaust Gas After Treatment using Air Preheating and Selective Catalytic Reduction by Urea to Reduce NO_x in Diesel Engine.” Heliyon, 11(1), 2025.