The premise is deceptively simple. Waste plastic, when heated in the absence of oxygen through pyrolysis, breaks down into a liquid hydrocarbon oil that resembles diesel in many respects. Rather than letting plastic waste accumulate indefinitely, this process recovers energy that was locked inside the polymer chains. The catch is that plastic-derived oil does not perform identically to petroleum diesel, and running an engine on it pure can raise problems with combustion quality, fuel consumption and emissions. The research team therefore chose a pragmatic blend: 90 percent conventional diesel mixed with 10 percent waste plastic oil, a formulation they labeled DP10. Into this blend they then introduced hydrogen gas at the intake, at rates of 5, 10 and 15 liters per minute, creating three additional test fuels.

The testbed was a single-cylinder, four-stroke direct-injection diesel engine running at a fixed speed of 1500 revolutions per minute under maximum load. What made the study distinctive was the second variable the team controlled: compression ratio. By adjusting the engine between compression ratios of 15 and 19, they could probe how squeezing the air-fuel mixture harder or softer changes everything from cylinder pressures to nitrogen oxide output. Compression ratio matters enormously in a diesel engine because ignition occurs spontaneously when compressed air gets hot enough; raising the ratio generally means faster ignition, higher peak pressures and, often, better thermal efficiency.

To make sense of the many possible combinations, the researchers turned to response surface methodology, a statistical optimization technique that builds a mathematical model of how multiple inputs jointly affect outputs. Instead of running every conceivable experiment, response surface methodology uses a carefully designed subset of runs to fit equations that predict brake thermal efficiency, brake-specific fuel consumption, nitrogen oxides and carbon dioxide emissions across the whole parameter space. This approach, widely used in engineering design, allowed the team to identify optimal operating regions and quantify interactions, for example whether the benefit of adding hydrogen grows or shrinks as compression ratio increases.

The results on performance were encouraging. As hydrogen enrichment increased, brake thermal efficiency climbed while brake-specific fuel consumption fell. Hydrogen carries a very high energy content per kilogram and burns rapidly, so even modest amounts mixed into the intake air help the diesel combustion process release energy more completely and quickly. The waste plastic oil blend, meanwhile, burned without catastrophic penalties at the 10 percent level, suggesting that small fractions of pyrolysis oil can be absorbed into existing diesel infrastructure without major engine modifications. Higher compression ratios pushed in-cylinder pressures upward at full load, consistent with the thermodynamics of denser, hotter charge air, which improves the conversion of fuel energy into useful work.

Emissions told a more nuanced story. Carbon dioxide output decreased when hydrogen was added, which makes intuitive sense: hydrogen contains no carbon, so every liter per minute of hydrogen that displaces diesel fuel removes a slice of the carbon that would otherwise oxidize to CO2. For anyone hoping to chip away at transport-related greenhouse gas emissions using existing engines, this is an appealing mechanism, particularly if the hydrogen itself is produced from renewable electricity rather than fossil feedstocks.

But there was a significant trade-off lurking in the exhaust. Nitrogen oxide emissions, the pollutants responsible for smog formation and respiratory health problems, rose considerably with hydrogen enrichment. The reason lies in combustion temperature. Hydrogen burns hotter and faster than diesel, and the higher in-cylinder temperatures encourage nitrogen and oxygen in the intake air to combine into NOx. This is a well-known dilemma in hydrogen dual-fuel diesel research, and the new study confirms it in the specific context of plastic-oil blends across a range of compression ratios.

Perhaps the most practically important finding was that more hydrogen is not always better. The researchers observed that going beyond 10 percent hydrogen enrichment, the 15 liters per minute case, had a negative impact on emissions, eroding the environmental gains that lower enrichment levels delivered. In other words, the relationship between hydrogen flow and overall emissions performance is not monotonic; there is an optimum near the middle of the tested range where the efficiency benefits and CO2 reductions outweigh the NOx penalty, and pushing past it tips the balance the wrong way. That kind of threshold information is exactly what engine calibration engineers need when deciding how much hydrogen to inject in real-world systems.

The study arrives at a moment when the internal combustion engine refuses to fade quietly. Despite rapid growth in electric vehicles, battery and charging infrastructure limitations mean conventional engines will dominate many markets for the foreseeable future, particularly in freight, agriculture and regions where electrification lags. Making those engines burn cleaner, partially renewable fuels is therefore not a nostalgic detour but a pragmatic climate strategy. Waste plastic oil adds a double dividend: it diverts persistent plastic waste from the environment while substituting a fraction of fossil diesel, and hydrogen enrichment squeezes more useful work out of every drop.

Caveats remain. The experiments were conducted on a single-cylinder research engine at one speed and full load, so real-world driving conditions with their constant load and speed variations would need further validation. Scaling up hydrogen supply, ensuring safe storage on vehicles and producing green hydrogen at low cost are all unresolved challenges. Yet the study’s methodical use of response surface methodology across compression ratios provides a reusable framework: rather than trial and error, future researchers and engine developers can use such statistical models to pinpoint operating windows where plastic-derived fuels and hydrogen coexist beneficially. As plastic waste mountains grow and decarbonization deadlines loom, work like this suggests that yesterday’s trash and tomorrow’s hydrogen might together keep today’s engines running a little cleaner.

Subject of Research: Optimization of hydrogen-enriched waste plastic oil and diesel blends in a variable compression ratio diesel engine using response surface methodology

Article Title: Analysis and optimization of waste plastic oil–diesel blend with hydrogen using response surface methodology at different compression ratios on a diesel engine

Article References: Rajak, U., Suresh, Y., Panchal, M., Verma, T. N., & Cuce, E. (2026). Analysis and optimization of waste plastic oil–diesel blend with hydrogen using response surface methodology at different compression ratios on a diesel engine. Clean Technologies and Environmental Policy, 28(9), Article 243. https://doi.org/10.1007/s10098-026-03594-6

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03594-6

Keywords: waste plastic oil, hydrogen enrichment, diesel engine, compression ratio, response surface methodology, brake thermal efficiency, NOx emissions, carbon dioxide, pyrolysis, dual-fuel combustion, alternative fuels, Analysis