Foods for thoughts: Diesel engines matches electric motors in a climate perspective!

Summary

Diesel engines in global freight is a main pillar that remains a valid solution, from energy efficiency,  climate impact mitigation and air pollutants perspectives. This somewhat controversial statement is explained in this brief “foods for thoughts-article” related to the target of moving forward towards climate neutral transport solutions. Below calculation describes the basis of the article.

In summary, results indicate that:

Diesel engines in trucks remains a valid energy efficient transport solution in the global market where electricity is produced by fossil fuels in processes without heat recovery. In those regions diesel (and otto if gas) engines running on renewable fuels are more climate efficient transport solutions.

Electric trucks, if based on electricity produced efficiently and based on renewable sources is the best solution from a climate, energy efficiency and local air pollutants perspective. If electricity is based on coal and at low production efficiency, diesel engine solutions is the better option.

Long term, the aim must be to replace finite energy resources with renewable sources which depends on regional abilities with regard to electrification options. Here hydrogen in combustion or fuel cell and electric motor may become the global solutions when sufficient energy efficiency is accomplished in these processes.

Essential is to ensure the ability to scale the use of all fuels, considering both finite and renewable fuels origin, environmental effects and ability to scale in a sustainable way. At present global consumption of roughly 100 million barrels of crude oil per day, the replacement by other options will in-evidently include “stepping stone” solutions not fulfilling all long-term requirements. They must to a certain extent however be allowed if tomorrows fully sustainable long-term solution will ever come true.

Background

Previous air polluting drawbacks of compression engines, known as the diesel engine includes the cumbersome tail pipe fumes of nitrogen oxides, particular matters and polyaromatics. Over the years they have led to severe negative health impact, in particular densely populated areas. Latest engine emission standards and low sulphur diesel fuel have however led to substantial emission reductions of air pollutants. The emission standard, Euro 6 allows NOx-emissions of maximum 0.4 g/kWh whereas European emission standard legislation from 1982 allowed 18 g/kWh.

Previously the global average of sulphur content in road diesel was 350 ppm (parts per million). There is presently a global trend towards lowering the sulphur content in diesel fuels. Read more. In the early 1990´s common diesel for road vehicles in Europe had a sulphur content well above 50 ppm. Today European countries apply diesel below 10 ppm sulphur. The effect is less sulphur oxide as well as less particular emissions. Still, emissions of small particles, partly related to diesel engines remain a challenge that needs further improvements.

In summary, general technology and legislation in combination have made substantial progress with regard to reduction of air pollutants from diesel engines. With this environmental progress in mind, this article aims to mirror the presently much more debated and challenging issue of how to reduce emissions of greenhouse gases in road transport.

Energy efficiency is key

Using less fuel by increasing energy efficiency is always a valid strategy. Not the least to save costs in the present very turbulent energy market. In this context electric motors are often referred to as more energy efficient. This is true when comparing vehicles in a tank-to-wheel perspective. The fact for relevant comparison is however the need to compare the two propulsion systems in a well-to-wheel perspective.

Assuming electricity generation based on oil in a gas turbine without heat recovery. The energy efficiency delivering electricity will be lower than the system including a combustion engine running on diesel. If the gas turbine system includes heat recovery it will be more energy efficient than the diesel fuelled vehicle. Hence, the obvious conclusion is a need to produce electricity in an energy efficient way. If gas turbine is used for electricity production this should be done in combined processes where excess heat is used for other purposes. Such solution leads to a very high efficiency rate. Other energy efficient ways to produce electricity are hydro power and wind power. Those systems involve other negative environmental impact that is difficult to assess in a non-biased way and not further explored here. The option of nuclear power production is even more difficult to asses in this context. Below calculation describes the main assumption basis of the article.

As can be seen in above figure, the diesel vehicle will remain a relevant solution in market where electricity is scarce or produced inefficiently.

Climate efficiency

The issue of greenhouse gas emissions is much dependent on primary energy used. If coal is used as fuel for electricity production there is both a risk of lower energy efficiency as described above but worse, that emissions of greenhouse gases will increase in comparison with the diesel engine. Below is a calculation example based on combustion engine and electric motor.

Key criteria´s for accomplishing GHG-emission reductions are use of energy carriers with low climate intensity.

Conclusions

Diesel engines in truck remains a valid energy efficient transport solution in the global market where electricity is produced by fossil fuels in processes without heat recovery. In those regions diesel (and otto if gas) engines running on renewable fuels are more climate efficient transport solutions.

Electric trucks, if based on electricity produced efficiently and based on renewable sources is the best solution from a climate, energy efficiency and local air pollutants perspective. If electricity is based on coal and at low production efficiency, diesel engine solutions is the better option.

Long term, the aim must be to replace finite energy resources with renewable sources which depends on regional abilities with regard to electrification options. Here hydrogen in combustion or fuel cell and electric motor may become the global solutions when sufficient energy efficiency is accomplished in these processes.

Essential is to ensure the ability to scale the use of all fuels, considering both finite and renewable fuels origin, environmental effects and ability to scale in a sustainable way. At present global consumption of roughly 100 million barrels of crude oil per day, the replacement by other options will in-evidently include “stepping stone” solutions not fulfilling all long-term requirements. They must to a certain extent however be allowed if tomorrows fully sustainable long-term solution will ever come true.

Stockholm 2022-07-04

Magnus Swahn