UPB tests hydrogen-diesel engine efficiency using mass flow meters

In an effort to reduce carbon emissions from the transportation sector, the European Commission is working to reduce CO2 emissions from transportation by 37.5% over the next 10 years. A promising approach is to focus on developing alternative fuels and technologies to replace greenhouse-intensive ones. If spark-ignition engines were to be replaced by clean diesel engines, CO2 emissions could decrease by 15% and overall fuel consumption would be reduced.

Clean diesel engines are, however, more complex and expensive than spark-ignition engines. To support the technology in its transitionary period, it is critical to develop sustainable and affordable fuels in the meantime. One such solution is to mix hydrogen fuel with traditional diesel fuel. Many groups are successfully implementing this solution, including the University of Politehnica of Bucharest (UPB). Here, we will evaluate their research into supplementing diesel fuel with hydrogen.

Research findings

The team’s primary research goal was to study how the addition of hydrogen to diesel fuel affects its energetic performance. They found that supplementing the diesel engine with hydrogen led to a 5.3% increase in engine efficiency while reducing the CO2, hydrocarbon, NOx, and smoke emission levels. These findings reinforce the effectiveness of using hydrogen as a way to make diesel fuel cleaner and more efficient.

Achieving the optimal mix of diesel, hydrogen, and air to provide oxygen for the reaction is a delicate balance of maximum pressures, combustion temperatures, and thermal efficiency across the range of the engine load.


Hydrogen is a promising way to boost diesel engine efficiency without the need for significant changes to engine design. However, scaled hydrogen implementation comes with challenges.

The first challenge is that hydrogen doesn’t mix well with diesel fuel, and storing pure hydrogen requires high pressure or cryogenic conditions. Hydrogen also has a high propensity to leak due to its small size and low density. Leaks are both expensive and put the safety of operators at risk. Finally, switching to hydrogen is only truly efficient if the hydrogen is sustainably produced. Using hydrogen produced from fossil fuels without carbon capture and utilization would ultimately negate the benefits.

The concept of increasing diesel engine efficiency seems attractive, however, it is not a quick and easy solution. Effective hydrogen implementation will be costly and require a lot of effort, reducing the likelihood of wide-scale adoption. This, in addition to problems of fossil fuel depletion and various forms of carbon taxing, means it may be more practical to invest in a pure renewable-powered system.

Testing setup

During the experimentation, an Alicat mass flow meter was used to monitor the flow of the hydrogen line into the engine test stand. This unit was used in conjunction with a hydrogen injector, alongside air and diesel flow control in order to obtain the desired diesel-hydrogen-air mix ratios. A 0-20% hydrogen supplementation was tested at 5% increments across a range of engine loads to determine the efficiency of the various mixes across the engine running conditions while also monitoring the engine pressures. The attached gas analyzer then allowed the analysis of the emission levels and constituents across the operating conditions.

A novel setup was used for efficiency testing under various operating conditions. The PLC controlling infrastructure communicated with the various instruments to ensure accurate fuel ratios. The diesel fuel cyclic amount was reduced by increasing hydrogen flow in order to keep the output brake power at the level of standard fueling. In this way, different hydrogen flows were set, in order to modify the energetic substitute ratio. Thus, ensuring the best correlation between engine running regime, fuel cyclic quantities, in-cylinder peak pressure, pollutant emissions levels, and exhaust gas temperature for high engine efficiency when using hydrogen.

Instrumentation for engine testing

Steady and reliable measurement of operating conditions is paramount to achieve reliable results when engine testing. Millisecond flow reading, ±0.1% repeatability, and integration with a range of communication protocols allow smooth and straightforward testing. Measurement ranges down to 0.01% of the instruments full scale combined with 98+ pre-programmed gas calibrations and included COMPOSER firmware to store 20 custom-defined gas mixtures are also valuable assets during the experimentation phase of engine R&D. This eliminates the reliance on correction factors (k-factors) when switching gases and reduces the number of lines and instrumentation sets required to cover wide operating ranges. Additionally, the constant visual feedback of line flow and pressure from the built-in unit display can be used to easily alter setpoints and gas mixes locally with the front panel controls.

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