Can diesel cars meet Euro 6 with water injection alone?
Most types of diesel fuel auto ignite below 300 Celsius, a temperature easily attained by compressing air in a car diesel engine having a compression ratio of say, 10:1. Fuel is sprayed into each cylinder near top dead centre when the temperature due to the compression is highest. The fuel detonates producing even hotter gases comprising mainly carbon dioxide and water. Any excess air not used in combustion is also heated. This increases the pressure even further. The increase in pressure generated by this chemical reaction in the cylinders is responsible for the power (or more accurately, the torque) produced by the engine. Unfortunately the high temperature causes oxygen and nitrogen in the excess air to combine to produce nitrogen oxides, commonly called NOx.
The European Emission Standards due to be imposed on engine manufacturers September 2014 (Euro 6)will set a NOx limit of just 0.08 grams per kilometre for new diesel cars, (currently 0.18) though carbon dioxide levels, the main greenhouse gas is exempted from this regulation. It will be a relatively easy process for manufacturers to achieve any low NOx limit simply by asphyxiating the combustion within the cylinders. By diverting a large percentage of the exhaust gases into the air intake, oxygen levels will be so low that there will be no opportunity for NOx to be produced. (Engineers call this “Exhaust gas recirculation”, EGR for short)
I have modified a diesel car by removing the catalytic converter and turning off the exhaust gas recirculation. This car can now average over 70 mpg, up from 50 mpg under the same driving conditions. Since carbon emissions are directly proportional to fuel used, this simple experiment highlights the damage to the environment caused by emission devices. Euro 6, shamefully will not concern itself over total greenhouse gases. However, I have a possible solution to limit both NOx and carbon emissions. Read on –
NOx is produced if the temperature within the cylinder spikes above about 1,250 Celsius. It would seem logical that if deionised water / fuel emulsion were injected into the cylinders instead of neat diesel, the latent heat of water would reduce the combustion temperature to below the critical 1,250 Celsius. (Latent heat causes that cold sensation when you get out of a bath). This cooling will drastically reduce NOx emissions. There will be further advantages. Superheated water within the diesel fuel will explode when injected, rather like the spitting in a damp frying pan. This will further atomise the fuel thereby assisting in better combustion and a reduction of soot in the exhaust. Secondly the water will be turned to gas and contribute to the cylinder pressure. This will increase the power of the engine. This result is, in theory, lower fuel consumption and lower carbon dioxide emissions. Let’s take a few ball-park figures to test this theory.
Diesel molecules are ‘paraffin’ chains around 50 carbon atoms long saturated with about 100 hydrogen atoms. To simplify my argument I will use ‘moles’, a little easier to imagine than molecules. A mole is a large but constant number of particles.
I mole of diesel weighs about 650 grams, 250 moles of water weigh 450 grams.
This ratio, 1.4:1 fuel to water, will still combust in most diesel engines. Bigger injectors will have to be fitted to restore power.
1 mole of diesel when burnt produces about 50 moles of water and 50 moles of carbon dioxide; 100 moles of gas particles altogether.
With water injection, I mole of diesel + 250 moles of water produces 350 moles of gas within the cylinders. (300 moles of water vapour and 50 moles of carbon dioxide)
The more gas particles, the greater the pressure in the cylinders, the more pulling power from the engine.
With about 3.5 times the number of gas particles in the cylinders after combustion, water injection should increase the pressure significantly leading to an improvement in the specific fuel consumption. A lower combustion temperature will allow a higher compression ratio and further efficiency improvements. Since peak temperatures will be lower it would be possible to cap the inside of the cylinder head and piston crowns with ceramic insulator – possibly incorporating a platinum catalyst able to further remove traces of NOx. Keeping the heat energy within the engine will lead to efficiencies, particularly at low revolutions. Current emission devices waste some 20 percent of the fuel which needs to be injected into the catalytic converter to regenerate the catalyst. Back pressure also increases pumping losses. Eliminating these losses caused the improved economy in the car I modified. It can be seen that water injection could, in theory substantially reduce NOx AND improve economy.
Water injection would require a sophisticated engine management system and strict monitoring of cylinder temperatures to ensure that these lie between 300 and 1250 Celsius. The ratio of fuel to water will have to be adjusted accordingly. Water will need to be mixed with the fuel just before injection into the cylinders and larger injectors will be needed to provide flow for the extra fluids. Also the injection timing will need to be advanced to compensate for delay in combustion in the cooler cylinders. A plastic water tank, 75% the size of the fuel tank will be needed (unless water can be condensed from the exhaust and recycled.) A detergent to emulsify the mixture, and ammonia (or urea) to neutralise acids produced in the combustion and to maintain a reducing environment will need to be added to the water prior to injection.
Water injection is a recognised way of increasing power in both reciprocating and jet aircraft engines. It would appear that NOx reduction could be a useful spin-off to this technology.
I am limited in this short blog; the minutiae of the technicalities required to make water injection practical would bore most people. You, dear reader have been very patient to plough on to the end. I thank you.
One final thought – “Why has this not done before?”
Post Script – this now employed on many new cruise ships and liners such as the Queen Mary 2
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