Reinventing diesel

Is there any way to change the petrol fractions of crude oil so they can work in a diesel engine?
26 July 2016

Interview with 

Professor Mani Sarathy, King Abdullah University of Science and Technology (KAUST)


Diesel and petrol engines work in different ways, which means that they need totally different forms of fuel. But here's the problem. Engineers and petro-chemists are predicting that, within the next decade or two petrol will no longer exist. We'll all either be behind the wheel of an electric car, or driving large transporters powered by diesel.

So what happens to the 20% of crude oil chemicals that currently go into making petrol? You can't just add them to diesel fuel because the present generation of engines can't burn them. So researchers at the King Abdullah University of Science and Technology in Saudi Arabia are developing new forms of fuel - and new diesel engine designs - that can. Combustion chemist Mani Sarathy showed Chris Smith around...

Mani - So we're working closely with industry here in Saudi Arabia, as well as around the world, to look at how we can maximise the usage of fuels that are generated from a barrel of crude oil. Right now, crude oil is used to make gasoline fuel for light duty vehicles and diesel fuel for heavy duty vehicles

In the future we see that light duty vehicles will be running probably on electricity. As a result, there's going to be an oversupply of gasoline. And our goal, as a research centre, is to make sure that fuels like gasoline (we call them naftas) can be utilised in diesel, heavy duty applications.

At the refinery side we actually want to minimise the amount of refining steps because the less refining you do, the less energy intensive it is. But less refined fuels will also not work in a conventional diesel engine so, at the same time, we are trying to promote a new type of diesel engine application - we call it a partially pre-mixed compression ignition engine. The diesel engine operates in a slightly different mode so that it can burn a larger variety of fuels.

The first step is we need to understand what the engine wants. For that to happen we really need to understand the combustion process in the engine. So these new types of pre-mixed charge, compression, ignition engines require fuels to autoignite with some specific properties, and we call that autoignition property the ignition delay time. So, if we know this parameter, then we can say how can we formulate the fuel to provide the ignition delay time that the engine wants?

Chris - Is that what you're doing in here?

Mani - Yes. So my colleague operates a shock tube lab, looking at how fuels would behave, different types of fuels at different temperatures, different pressures, and varying mixing fractions of fuel and air.

Chris - Shall we take a look?

Mani - Sounds good...

Chris - This is a big room and there are a sequence of very, very large stainless steel pipes. They're what - 6 inches in diameter pipes stretching the length of the room. What are they - what do you do with them?

Mani - The pipes are roughly separated in half. Four metres is what we call the drivers section of the shock tube; four metres is the driven section, and these two sections of pipe are separated by a diaphragm. This is the diaphragm; it's made of aluminium and on the diaphragm there's a score. The score is to facilitate the bursting of the diaphragm.

Then on the driver section an inert gas is injected up to a very high pressure and then we apply a slight prick to the diaphragm. As the diaphragm bursts, a shock wave will be accelerated down the second half of the tube. In the second half of the tube there's a mixture of fuel and air. As the shock wave travels down it compresses the test gases and then the shock wave hits the end of this tube and reflects back, and as it reflects back that reflected shock further compresses this fuel/air mixture.

There is a series of pressure sensors to determine the shock speed; also to determine what is the pressure of the test gas section.

Chris - What about the chemical composition of what's going on in there - can you see that going on too?

Mani - Absolutely! So, as the reflected shock goes back and heats up the fuel/air mixture, the fuel starts to oxidise and undergo various reactions. These can be captured by a number of diagnostic techniques based on lasers.

Chris - And by shining laser light in, are you relying on the fact that certain colours or wavelengths of laser light will be absorbed by certain chemicals that are being produced by the reactions?

Mani - That's pretty much what happens. Different molecules are going to absorb different wavelengths of light. So we have a spectral database which tells us: CO2, hydrogen peroxide, carbon monoxide, different types of hydrocarbons which are the intermediates of a combustion process. Then you have detectors tuned to detect light coming out at the other end. And then it's a simple relationship (what we call Bio-Lambert's law), which relates the absorption with the concentration of the gas in the test section and we can monitor this over about a one microsecond.

Chris - The thing is a big 6 inch caliber stainless tube is not however an engine...

Mani - It's not replaceable for the real thing but what it's really good for is making sure our models work well. Now we have accurate chemical kinetic models for our fuels. We can take the fuel formations which we have tested here and we can go and test them in our engine facility...

Chris - Well it certainly smells like an engine room in here - that beautiful oily smell that people like fiddling cars will be acquainted with. We've got two big engines in here and what's the difference between the two?

Mani - One is primarily used for a gasolene engine, so it's a spark ignition engine and the other is meant mainly for diesel engine research as a compression ignition engine.

Chris - And they're all rigged up with a whole panolply of different sensors so that you can monitor the conditions going on?

Mani - Yes. So just like in the shock tube lab we were very concerned with the temperature, the pressure, and the mixture, and how that affects the ignition process. The same way in an engine you've got to control these three parameters: we have compressors to control the pressure intake, we sometimes have heating jackets to control the temperature of the intake stream, we can vary the engine speed to determine the temperature inside the cylinder, the pressure inside the cylinder and, by varying the amount of fuel injected, we vary the mixture fraction inside the cylinder, the mixture stratification. We also measure the emissions that you would be concerned about coming from your engine.

Chris - And then what - do you go back to the automotive manufacturers and say right, look, we know what the fuels can do, this is what you need the engines to do to accommodate these fuels?

Mani - That's exactly right. And there's also an engine research community that's helping to develop new types of engine combustion modes and the research community has developed these engines and said these are the types of engines that are more efficient, cleaner burning, these are the combustion engines of the future. In order for industry to adopt them, industry needs to be comfortable, not only that they can make these engines, but they're also going to have the fuel that these engines require.


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