Overlapping Diesel and Gas Injections Push Natural Gas Truck Engines to Record Efficiency

Heavy-duty trucks are among the hardest vehicles to decarbonize, and a new experimental study suggests that a surprisingly simple change in how two fuels are timed inside the cylinder could make a real dent in their carbon footprint. Researchers working on an 11.59-liter, six-cylinder high-pressure direct-injection natural gas engine have shown that deliberately letting the diesel pilot injection and the natural gas injection overlap in time, rather than keeping them strictly sequential, can push brake thermal efficiency to approximately 46.5 percent, a gain of 1.7 percentage points over the conventional injection scheme. The finding, published in Results in Engineering, comes at a moment when natural gas heavy trucks are booming in key markets: China alone recorded 178,200 natural gas heavy truck sales in 2024, a 17 percent year-on-year increase, and the national fleet surpassed 900,000 units by mid-2025.

The engine architecture at the heart of this work is known as diesel micro-pilot ignition high-pressure direct injection, or HPDI. In the classic configuration, a tiny quantity of diesel, held here at just 5 percent of the total fuel energy, is injected first to create multiple ignition cores. Only after the diesel injection ends does the high-pressure natural gas jet enter the cylinder, where it ignites on those fire kernels and burns in a diffusion flame. Both injections happen close to top dead center, and because the natural gas never premixes with air before ignition, the engine avoids the knocking problems of spark-ignited gas engines while retaining diesel-like efficiency. Under China VI emissions standards, optimized HPDI engines have already exceeded 54 percent brake thermal efficiency, making them serious competitors to pure diesel powertrains.

What distinguishes the new study is the breadth of timing combinations explored on a mature dual direct-injection test bench. The team fixed the engine at its economic operating point of 1300 revolutions per minute and 1400 newton-meters of torque, then swept the diesel start of injection across nine positions for each of four natural gas injection timings: 13.8, 15.8, 17.8 and 18.8 degrees of crank angle before top dead center. This produced injection intervals, denoted DNI, ranging from minus 10.4 to plus 8.1 crank-angle degrees, spanning everything from fully sequential injection to conditions where natural gas actually enters the cylinder before the diesel. When the interval reaches minus 5.2 degrees or more, the researchers call it a compound injection mode, because the two fuel events overlap or even reverse their traditional order.

The efficiency results tell a clear story. In the classic sequential mode, shifting the timings changed brake thermal efficiency by less than half a percentage point, peaking at 44.8 percent. In the compound mode, however, delaying the diesel injection toward top dead center produced a pronounced rise-and-fall pattern. The warmer in-cylinder environment near top dead center helps the diesel atomize and evaporate, while the growing premixed fraction of natural gas and a more locally concentrated diesel-rich zone around the ignition region together favor rapid, well-phased heat release. Advancing the natural gas injection pushed the heat release earlier in the expansion stroke, further improving the engine’s ability to extract work. The optimum arrived at an injection interval of 6.1 crank-angle degrees, where efficiency climbed to roughly 46.5 percent.

The mechanism behind that optimum is a delicate balance of mixture physics. With a positive injection interval, natural gas enters first and has time to premix with air, while the late-injected diesel stays concentrated near the ignition zone, where it does the most good. Push the interval too far, though, and the early-injected gas migrates toward the cylinder walls, where flame propagation is poor and oxygen is scarce, dragging combustion efficiency back down. The diesel energy fraction remained fixed at 5 percent throughout, so the gains cannot be attributed to simply burning more pilot fuel. Instead, the study demonstrates that the spatial distribution of that small diesel charge, and its interaction with the surrounding gas jet, can be tuned like a combustion-control dial.

Emissions, inevitably, complicate the picture. Nitrogen oxide emissions rose by about 53 percent on average as the diesel injection was delayed, reaching a maximum of 18.4 grams per kilowatt-hour at the most advanced natural gas timing. The engine runs lean, with an excess air coefficient near 2.0, so its nitrogen oxides are almost entirely thermal in origin, formed where temperatures spike. Shorter main combustion durations concentrated the heat release into smaller, hotter zones, and advancing the gas injection raised peak cylinder pressures and moved them closer to top dead center, both of which favor nitrogen oxide formation. Unburned hydrocarbon emissions climbed even more steeply, increasing roughly 134 percent on average and peaking at 1.1 grams per kilowatt-hour, because excessive injection intervals sent more natural gas into the near-wall and crevice regions where it escapes complete oxidation.

Carbon monoxide followed a more intricate path, tracing the strength of the interaction between the two fuel jets. When the overlap angle between diesel and natural gas injection was largest, the two sprays competed directly for oxygen, and carbon monoxide emissions responded strongly to the timing sweep. Outside that high-overlap window, carbon monoxide stayed relatively low and stable. Notably, at the highest-efficiency settings with large overlap degrees, the compound mode produced carbon monoxide levels roughly unchanged from the conventional mode, meaning the efficiency gain did not come at the cost of that particular pollutant. The overall trade-off, however, remains real: the compound injection mode delivered more nitrogen oxides and hydrocarbons than the sequential baseline, and the authors acknowledge that reconciling high efficiency with stringent emission control is still an open challenge.

The combustion diagnostics reveal why the two injection modes behave so differently. In the sequential mode, delaying the diesel injection sharply shortened the ignition delay, by up to 6 crank-angle degrees, because hotter conditions near top dead center accelerate diesel auto-ignition. In the partially overlapping compound regime, between minus 5.2 and plus 3 degrees, the ignition delay stubbornly held at about 18 degrees regardless of either timing, because the cold, low-reactivity gas jet dilutes oxygen and chills the diesel spray, offsetting the igniting effect of the later diesel timing. Once natural gas led the diesel by more than 3 degrees, the shrinking overlap weakened this interference and ignition delay shortened again. Meanwhile, the main combustion duration contracted by up to 21 crank-angle degrees across the sweep, with the compound mode shortening it by as much as 19 degrees relative to the baseline, as a larger share of the gas burned in premixed fashion rather than in a slow diffusion flame.

Peak cylinder pressure added a practical bonus. As diesel injection was delayed past an injection interval of roughly zero, peak pressure fell sharply because the expanding cylinder volume during the expansion stroke absorbed the heat release, easing the thermal and mechanical loads on the engine. Synchronously advancing both injections produced earlier combustion phasing and higher peak pressures, confirming that the diesel timing dominates the combustion duration while the gas timing shapes the overall phasing. The authors caution that all tests were conducted at a single operating point, that the reported 1.23 percent efficiency uncertainty reflects instrument precision rather than full repeatability testing, and that future work will extend the campaign across the engine map and deploy numerical simulations to untangle the nitrogen oxide, hydrocarbon and carbon monoxide formation mechanisms.

Even with those caveats, the study lands at a consequential moment. The European Green Deal is tightening rules for medium- and heavy-duty vehicles, and natural gas, with its low carbon-to-hydrogen ratio and soot-free combustion products, is widely viewed as a pragmatic transitional fuel for freight. If a few crank-angle degrees of injection overlap can buy nearly two percentage points of efficiency on a production-relevant engine platform, fleet operators and engine makers have a low-cost lever to pull while battery-electric and hydrogen trucks mature. The next hurdle will be taming the nitrogen oxide penalty, likely through aftertreatment calibration or injection strategies that preserve the premixed fraction without concentrating heat release, but the demonstration that fuel-jet timing alone can reshape combustion this dramatically is a striking reminder that some of the biggest decarbonization wins are still hiding in the fine print of injector control maps.

Subject of Research: Injection timing strategies in diesel micro-pilot ignited high-pressure direct-injection natural gas engines for heavy-duty trucks

Article Title: Impact of the fuel injection phase on the combustion and emission performance of a heavy-duty diesel micro-pilot ignition high pressure direct injection natural gas engine

Article References: Jia, D., Lin, J., Wang, Z., Yang, K., Wang, D., Wang, X., & Wang, Z. (2026). Impact of the fuel injection phase on the combustion and emission performance of a heavy-duty diesel micro-pilot ignition high pressure direct injection natural gas engine. Results in Engineering, 32, Article 113441. https://doi.org/10.1016/j.rineng.2026.113441

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113441

Keywords: HPDI, natural gas engine, heavy-duty trucks, injection timing, brake thermal efficiency, diesel micro-pilot ignition, NOx emissions, THC emissions, combustion phasing, compound injection mode, decarbonization, Results in Engineering