There’s a new kind of engine quietly redefining what we might call “clean diesel.” It doesn’t run on a new fuel. Instead, it runs on an old one — quite literally.
In a recent study led by researchers at Umm Al-Qura University in Mecca, Saudi Arabia [21.4°N, 39.8°E], scientists have successfully blended waste lubricant oil — the kind drained from engines after thousands of kilometres — with hydrogen gas, a bit of nitrogen, and a dose of exhaust recirculation to see what would happen inside a diesel engine.
The result? A remarkable 33% improvement in fuel efficiency, a noticeable drop in toxic emissions, and a strong case for rethinking both what we throw away and what we put into engines.
This matters for anyone in Northern Europe or Canada — places where diesel engines are still crucial for powering trucks, agricultural machines, marine vessels, and off-grid generators, and where waste oils and emissions standards are part of a daily policy puzzle.
From Waste to Workhorse
The study focused on a common rail direct injection (CRDI) diesel engine — the same type used in millions of commercial vehicles today. Researchers replaced 20% of the diesel fuel with waste spent lubricant oil (WSLO), then enriched the mixture with hydrogen gas. They also experimented with adding nitrogen (N₂) and exhaust gas recirculation (EGR), which reintroduces cooled exhaust gases into the combustion chamber to reduce certain pollutants.
Despite the complexity, the goal was simple: boost engine performance while cutting emissions and repurposing a major waste stream.
Why Hydrogen? And Why Waste Oil?
Hydrogen is widely known as a clean fuel — it burns without producing carbon dioxide — and its high flame speed and energy content help ignite fuels more completely. But it’s often considered impractical for existing engines.
This study showed otherwise. Adding 25% hydrogen to the WSLO blend significantly reduced unburned fuel, increased combustion pressure, and raised exhaust temperatures — signs that the engine was working more efficiently, not harder.
The use of WSLO, meanwhile, addressed a different problem. Globally, more than 25 million tonnes of waste engine oil are produced each year, and much of it is poorly managed or discarded. By reclaiming and cleaning WSLO for use in fuel, the team found a way to reduce waste and emissions at the same time.
Key Numbers
When running on a blend of 20% WSLO and 25% hydrogen, the engine showed:
- A 33% drop in fuel consumption compared to regular diesel
- A brake thermal efficiency (BTE) of 44%, up from 37% for diesel
- An increase in peak cylinder pressure, a sign of stronger combustion
- A 25% reduction in hydrocarbon (HC) emissions
- A drop in carbon monoxide (CO) emissions by over 20%
However, things weren’t perfect. Adding nitrogen — used to suppress combustion and lower nitrogen oxide (NOx) emissions — came at a cost. When 13% nitrogen was added, NOx fell by 21%, but CO and HC emissions rose again, as the combustion became less complete.
This delicate balancing act is typical of engine research: improve one metric, and another often shifts. But with careful tuning, this blend achieved strong results in emissions reduction without sacrificing performance.
Cleaner Burning, Fewer Emissions, Smarter Design
One of the most compelling aspects of the study is how it combined several techniques — not just one — to fine-tune engine behaviour:
- Hydrogen helped achieve faster ignition and higher combustion pressure
- EGR lowered peak flame temperature, reducing NOx
- Nitrogen further cooled combustion but needed moderation
- WSLO offered real-world waste recovery, contributing to the circular economy
At full engine load — the hardest test for emissions — the hydrogen-WLSO blend outperformed standard diesel on nearly every measure. While adding nitrogen helped suppress NOx, it had to be kept in check to avoid backsliding on other pollutants.
Why It Matters for the North
This kind of flexible, multi-fuel strategy holds real promise for countries with a legacy of diesel dependence, especially where:
- Hydrogen is becoming more accessible
- Emissions standards are tightening
- Waste oils are plentiful, but underused
- Diesel engines remain essential for agriculture, freight, and power
In rural regions of Norway, northern Scotland, the Canadian Prairies, or even Arctic outposts, fuels must be stable, affordable, and compatible with existing engines. This study shows that — with some engineering insight — we may not need to throw away what we’ve already built. We just need to fuel it smarter.
A Bridge to the Future
The paper doesn’t claim to have found a silver bullet. But it does offer a compelling transitional solution — one that reduces carbon impact while making use of waste materials and existing infrastructure. It also opens doors to further innovations, such as hydrogen direct injection or alcohol–WSLO blends, already being explored by the same research team.
For now, the message is this: cleaner engines aren’t just a matter of switching fuels. Sometimes, they’re a matter of using what we already have — better.
Source
Combined impact of hydrogen fuel, nitrogen, and EGR with waste spent lubricant oil blended diesel on common rail direct injection diesel engine under varying loads, Journal of Umm Al-Qura University for Engineering and Architecture, 2025-02-06
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