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Using Hydrogen Injection to Improve Idle Combustion and Emissions Performance at Lean Conditions of Gasoline Engines

Gasoline and hydrogen rails and injectors installed on the intake manifolds. Credit: ACS. Click to enlarge.

Spark-ignited engines suffer high specific fuel consumption, emissions, and cyclic variation at idle and lean conditions. Increasing levels of hydrogen enrichment of a gasoline fuel charge in a spark-ignited (SI) engine can increase engine-indicated thermal efficiency and emissions at idle and extend the lean-burn limit, according to a study by researchers from the Beijing University of Technology. A paper on their work was published online 18 August in the ACS journal Energy & Fuels.

Changwei Ji and Shuofeng Wang modified a 4-cylinder (SI) engine to permit hydrogen and gasoline to be injected into the intake ports simultaneously to realize a hybrid hydrogen-gasoline engine (HHGE). The hydrogen and gasoline flow rates were governed by a hybrid electronic control unit (HECU).

Three hydrogen volumetric fractions of 0, 3, and 6% in the intake were applied to investigate the effects of hydrogen addition on engine thermal efficiency, combustion duration, cyclic variation, and emissions at idle conditions using a fixed spark advance.

Under each hydrogen enrichment level, the engine was gradually leaned by reducing the gasoline flow rate until the coefficient of cyclic variation in indicated mean effective pressure (imep) was above 10%.

The test results showed that, with the increase of hydrogen enrichment level, engine-indicated thermal efficiency was improved, and the lean burn limit was extended. The peak in-cylinder temperature and in-cylinder temperature at exhaust valve opening decrease with the increase of excess air ratio and hydrogen blending level.

For a specified excess air ratio, HHGEs tend to get a high residual gas fraction compared with a conventional engine. The combustion duration was gradually shortened with the increase of hydrogen addition level at diluent conditions. Moreover, CO and NOx emissions were reduced at lean conditions for the HHGE. HC emissions from the HHGE were also lower than the original one at all excess air ratios.

Specifically, the researchers found that:

  1. Under the same excess air ratio, indicated thermal efficiency of the test engine improves with the increase of hydrogen fraction. The relative excess air ratio for the maximum indicated thermal efficiency increases with the addition of hydrogen.

  2. At idle condition, due to the high flame speed, small ignition energy of hydrogen, and the increased OH radical after hydrogen enrichment, the flame development and propagation durations are shortened by the addition of hydrogen.

  3. Because of the shortened CA0-10 and CA10-90, COVCA0-10 and COVimep of the HHGE are lower than the original engine at the same λ. The engine lean burn limit is also extended after hydrogen enrichment.

  4. NOx emissions at idle condition are reduced by increasing hydrogen blending fraction. When the hydrogen-gasoline mixture combusts near stoichiometric excess air ratio, HC emissions from the 6% HHGE are slightly higher than those from the 3% HHGE but are still lower than those from the original engine.

  5. At the lean burn limit, the engine-indicated thermal efficiency is improved, whereas CO2, CO, and NOx emissions are reduced with the increase of hydrogen enrichment level. However, HC emissions at the lean operating limit are raised with the increase of hydrogen addition fraction, due to the further increased λLL, which causes lower combustion temperature and partial burning of gasoline.


  • Changwei Ji and Shuofeng Wang (2009) Effect of Hydrogen Addition on Idle Performance of a Spark-Ignited Gasoline Engine at Lean Conditions with a Fixed Spark Advance. Energy Fuels doi: 10.1021/ef900517t


Henry Gibson

Smaller engines make cars more efficient. Hybrid cars are more efficient. Hybrid cars with smaller engines are even more efficient. ..HG..

richard schumacher

Does the increased engine efficiency more than pay for the energy required to make the hydrogen?


"can increase engine-indicated thermal efficiency and emissions at idle...
I got a radical idea: turn the engine off when it's idling.


Nothing new here, we already know H2 injection helps a car lean burn. dursun has it right a car should have stop/start but H2 injection would still work at partial loads. But hey let's go all out and also do what Henry suggests; downsize the engine, use a turbo/supercarger to get the power back up and use direct ethanol injection so you can use high efficiency high compression.


Roger Pham

H2 injection can be used on top of engine downsizing and turbocharging, as well as electric hybridization (HEV), for even more gain in fuel efficiency. With enough H2 storage on board, for example, with future's super-duper H2 absorption tank to allow higher quantity of H2 storage, the car can run mostly on H2 during cruise at part load. It only needs gasoline injection during high load in order to gain more power, since H2 port injection will reduce the air volume, hence available O2 for combustion. Furthermore, port injection of H2 may not be used at high engine power with higher exhaust valve temperatures, since premature ignition will occur too easily at higher H2 contration.

Bottomline: This type of research will pave the way for H2 utilization in ICE in the near future. Coupled with engine downsizing and electric hybridization, H2 utilization at part loads can result in tank-to-wheel thermal efficiencies above 50%, perhaps closer to the performance of Fuel cells. The Prius II has tank-to-wheel efficiency of 37%. Prius III's performance should even be higher, may be ~40% tank-to-wheel efficiency.

Roger Pham

I must hasten to add that for near 100% utilization of H2 at part load, the engine compression ratio must be increased to 13 or 14, to take advantage of higher octane rating of H2, for maximum efficiency. In that case, instead of gasoline injection at higher loads, ethanol injection should be done, to take advantage of equally-higher octane rating of ethanol. Or, a variable-compression mechanism can be used if gasoline use is to be preferred, since gasoline is widely available, while H2 and ethanol are not widely available. The engine should already be running on Atkinson cycle instead of Otto cycle in order to maintain high efficiency.


Here is an even more radical idea. Why not use the hydrogen in a fuel cell to get rid of the gasoline outright.


Because gearheads LIKE engines. Fuel cells are fine if you're never going to open the hood and show off but sometimes owning a car is all about what the guy next door thinks when he hears it roar to life while he's still sitting down at the kitchen table having his morning OJ.

Plus, there are a lot of ICE cars out there that still have a lot of life left in them. They will need retrofitting.

Roger Pham

Thanks, ai vin. The throaty roar of a sport-car engine is music to our ears (gearheads').


Didn't on-board generated hydrogen injection units accomplished/claimed the same and more a few years back?

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