Compression-Ignition Engines - diesel engines

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Introduction

Diesel engines or internal combustion engines are used in stationary, marine and automotive applications. Direct use of pyrolysis liquids in conventional engines is not possible and either the engine or the fuel needs to modified. The use of unmodified pyrolysis liquid is only considered for stationary applications like Combined Heat & Power (CHP). For use as transportation/automotive fuel the liquids needs upgrading ( [1] Biofuel ).

In a diesel engine or compression-ignition engine incoming air is compressed in the cylinder. As a result of the compression the temperature of the air increases which should reach a value well above the self-ignition of the fuel. The fuel is sprayed (atomized) into the hot air and the fuel will immediately burn and deliver the work. An extensive overview on the development of these engine can be found elsewhere History of Internal Combustion Engines A very good overview paper on the use of pyrolysis oil in engines and turbines has been prepared by Chiaramonti, Oasmaa and Solontausta [1] .

CN number.jpg
Besides the modifications required due to the acidic nature of pyrolysis oil also its poor ignition characteristics should be considered carefully. For diesel engines the Cetane Number (CN) [2] is the relevant indicator, and mineral diesel has typically a CN-value around 50. A high CN value makes diesel engine application easier. For pyrolysis oil hardly any reliable CN value can be found and CN-values reported vary between 5 and 25 [2] or even lower.


Test engines

Several tests have been performed on various diesel engines from laboratory test units to large size modified dual fuel engines. Test results mention positive results of engine performance in terms of smooth running. Nevertheless some problems still need to be resolved to use bio-oil to replace diesel, especially if the acidic nature of the oil (pH ≈ 3) and its tendency for soot formation and re-polymerisation is considered. The use of a bio-oil requires modification of various parts of the engine, amongst which the most important ones are the fuel pump, the linings and the injection system. Slight modifications of both the bio-oil and the diesel engine can render bio-oils a quite acceptable substitute for diesel fuel in stationary engines (CHP applications).


Bio-oil tests with various sized diesel engines
Company Country Engine Remarks
Ormrod Diesels UK 250 kWe Mirrlees Blacksone, 6 cylinder medium speed Unknown modifications to the engine
VTT - Wärtsila Finland 4.8 kWe Petter AVB Coking of injector nozzle; clogging of the bore for cylinder pressure measurement
84 kWe Valvet DS 420 injectors modified
Wasa 18V32 able to withstand pyrolysis oil
Pasquali machine Italy 29 kWe Landini Initial testing with pure bio-oil; later emulsions of bio-oil and diesel; cooled injection system
University Rohstock Germany 300 kWe No results published
Pytec Germany 300 kWe Mercedes Benz 10 hrs continuous operation; diesel pilot injection
BTG Netherlands 20 kWe one cylinder 40 hrs continuous operation; tests on-going

Wärtsila

The fuel feeding and injection system is critical for fuelling pyrolysis oil in a diesel engine. Standard materials are not corrosion resistant and can not be used. Best materials in the tests of Wärtsila (mid nineties) were:

• Injection equipment: Martensitic sintered steel M390 (1.90% C, 20% Cr, 1% Mo, 4 % V, 0.6% W)

• Injector holders and body:X35CrMo17

• Pushrods and needles: X90CrMoV18 (1.4112, martensitic stainless steel 57 Hrc hardness)


Engine testing at BTG

BTG converted a standard, JIANG DONG, CI-engine to allow it to run on pyrolysis liquids.
Engine.jpg
Three fuel vessels are installed containing engine start-up fuel (mineral diesel), rinsing fluid (e.g. ethanol) and the pyrolysis liquid, respectively. The test fuel can be preheated to about 100 °C, and the incoming air can be controlled at temperatures between 20 and 220 °C. Two pistons were available, one corresponding to a compression ratio of 17.6 and another one to a ratio of 22.4. A generator connected to the engine converts the mechanical power into electricity. The electrical load was varied in 1 kWe steps between 1 kWe and 12 kWe. The original fuel pump and fuel injector were replaced by a complete, dedicated stainless steel fuel injection system. BTG constructed both parts in-house, as suitable suppliers for such parts could not be identified.


To overcome the poor ignition properties of pyrolysis oil a higher temperature is required in the engine cylinder when the fuel is injected. This can be achieved by increasing the air inlet temperature or the compression ratio. For pyrolysis oil fuelling an air inlet temperature of around 100 °C is required at a compression ratio of 17.6. By increasing this ratio to 22.4 the air inlet temperature can be reduced with 40 °C. Adapting the fuel injecting timing may have some advantages for pyrolysis oil fuelling. Early injection means that more time is available to ignite the fuel and to achieve complete combustion. It appears that the optimal timing for engine operation on pyrolysis oil is very comparable to that of sunflower oil and biodiesel. Early injection of pyrolysis oil resulted in severe operational problems.

In "duration tests", the CO emissions and pyrolysis oil fuel consumption were monitored when the engine was operated for a period of 40 hours, spread over several days (a couple of hours each day). In the first hours of operation the engine performance improved somewhat and then it stabilized. Obviously, for further development real long duration testing will be important. Other upgraded, pyrolysis derived bioliquids were also tested (e.g. blends, emulsions, mildly hydrotreated, esterified) were tested in the diesel engine. Generally, the upgraded liquids were easier to ignite resulting in lower CO emissions and higher NOx, whereas the effect on overall efficiency was less pronounced. The liquids remain acidic, and thus a modified injection system will be required in all cases.


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References

  1. Chiaramonti, Oasmaa and Solontausta, Renewable and sustainable Energy Reviews, 11, 2007, pp. 1056-1086
  2. The use of pyrolysis oil and pyrolysis oil derived fuels in diesel engines for CHP applications, L van de Beld, J Florijn, E Holle, Applied Energy 102, 2013, pp 190-197.