High pressure diesel cycle combustion

The common heavy duty high performance engine runs according to the diesel cycle. Over time the efficiency of these, especially the really large ones, have reached efficiencies as high as 50 % and above. The conventional fuel is oil, usually either refined marine diesel or marine gas oil with lower viscosity or some type of heavy fuel oil with higher viscosity and a higher degree of impurities solved in the oil. Generally, a larger engine at lower speed is able to burn heavier fuels.

For operation on methanol, several methods is possible.  

DME

Dimethyl ether (DME) in contrast to methanol is an excellent diesel fuel with high cetane number (>60) and reasonably high energy density (28,430 kJ/kg). DME is in essence dehydrated methanol; two methanol molecules are combined to produce one molecule of DME and one molecule of water. DME is gaseous in ambient conditions and require a pressure of about 5 bar to stay liquid but do not require cryogenic storage.

Figure 3: The chemical reaction where two molecules of methanol become one molecule of DME and one molecule of water. 

DME has in particular been identified as a potential energy carrier for trucks. Recently there has been some attention to a project from Volvo to use bio-DME from the pulp industry to fuel trucks in Sweden. The system employed by Volvo use DME in liquid form all the way from the fuel station to injection in to the cylinder. 

The combustion process is similar to ordinary fuel oil; DME is injected to the cylinder at TDC and is ignited by the high pressure and temperature. Compared to an ordinary production engine the required modifications should to a large extent be limited to the fuel supply and injectors. 

A hurdle for DME is the fact that it is gaseous at ambient temperature and pressure conditions. This requires pressurised storage tanks and also makes bunkering and demands on pipes and bunkering stations stricter as higher pressures is demanded. Also, as DME is not available as a worldwide commodity and no distribution network exist an introduction of it as a global fuel is difficult and expensive. 

OBATE

A closely related alternative to DME is called OBATE™ (On Board Alcohol To Ether). It is a system developed by Haldor Topsoe that converts methanol to DME on board of a ship. That way no distribution network for DME is needed as well as simplifications for bunkering and on board storage as methanol is liquid at ambient pressure and temperature. In the OBATE system methanol is converted to a mixture of DME, water and methanol that is used as fuel in the engines. 

Pressure pipes are still required between the OBATE-system and as well as required modifications to the fuel system. As the fuel have better self ignition properties compared to raw methanol extensive modifications to the engines might be avoided. 

A pilot installation of an OBATE system has been tried in the SPIRETH project. Two Scania DI13 auxiliary engines and an OBATE reactor have been installed on board Stena Scanrail, a Ro-Ro vessel in traffic between Gothenburg and Frederikshavn.

Preliminary results from the projects indicates that, in contrast to DME, OBATE is unlikely to function as well in a diesel cycle without further modifications to the engine. The water and methanol content in the fuel lowers the cetane number and modifications such as higher compression ratio and/or ignition enhancer might be needed for an engine to work well at all running conditions on OBATE. 

The system also requires room for the reactor. Based on the installation on board Stena Scanrail the required space would likely be somewhat larger than the separator room required for heavy fuel operation [1]. Further development of the OBATE reactor will significantly reduce the size.

Methanol diesel HPDI (High pressure direct injection)

Instead of converting methanol to DME it could be much more attractive to use methanol as it is. The high pressure diesel concept relies on direct injection of methanol to the cylinder much like the ordinary diesel cycle. 

In order to guarantee proper ignition a small burst on pilot diesel fuel is injected first. This first burst of pilot fuel ignites and raises the temperature in the cylinder before an injection of methanol follows. 

 

Figure 4: The combustion principles of pilot diesel injection: 1) The piston compresses air in the cylinder. 2) At the end of compression a pilot injection of diesel fuel is injected to the cylinder and combustion is initiated. 3) Methanol is injected to the ongoing combustion. 4) Exhaust gases is vented from the cylinder. 

Emissions of particles is heavily reduced and limited to the pilot diesel. NOX emissions are expected to be low, in line with LNG operation. Formaldehyde forming is unlikely. 

As methanol is injected to the ongoing combustion at TDC the cylinder liner and air channels are not exposed to methanol. Likewise blow-by gases should not contain methanol that could increase decomposition of lubricating oil in the crank case. Modifications to the engine should be limited to the fuel injection system with full flexibility to operate on conventional diesel fuel with no loss of performance. 

The fuel injectors can be designed in different ways. Either by using separate injectors for methanol and diesel or by use of an injector type capable of distributing both types of fuel separate of each other in the same unit. 

Figure 5: Principle of fuel injector capable of distributing two separate fuels individually. The model is based on a fuel injector from Westport. 

The injection mechanism can be controlled in different way. Oil, solenoid or piezoelectric controller can all be found in commercial products. 

Wärtsilä Z40S concept as applied on Stena Germanica

The methanol diesel concept will be used on a conversion of Stena Germanica to methanol. The Z40S engine modifications are based on an existing gas-diesel design that has been further developed for methanol. The combustion principle is a methanol diesel according to the method previously described 

High pressure 2-stroke concept (B&W/MAN concept)

The ME-LGI (liquid gas injection) two stroke engine from MAN Diesel & Turbo is capable of dual fuel operation according to the diesel cycle. The engine is based on the proven ME-GI (gas injection) concept that has been developed since the early 90’s and is in use on both sea and land. When operating on gas or methanol a burst of pilot diesel is used to initiate the combustion. The cylinder head is fitted with two fuel oil valves and two gas valves that can be exchanged for valves suited for methanol. A common rail system is used to supply gas and oil controlled valves is used to control the injection timing. Similar to the high compression diesel the risk of methanol contamination is very low as the fuel is injected to the ongoing combustion. The arrangement provides full flexibility to shift seamlessly between different fuels with no loss of performance. For prolonged operation on diesel fuel it is recommended to exchange the special fuel injectors for regular ones to increase fuel efficiency. 

  

Figure 6: Diesel cycle combustion in a 2-stroke engine: 1) Scavenging, air inlet in bottom of cylinder is open and exhausts are vented through the open top valve. 2) Air is compressed by the piston. 3) Pilot injection of diesel at TDC, combustion begins. 4) Methanol is injected to the ongoing combustion.

The ME-GI concept for methanol will be used in six announced methanol tanker newbuildings that will be on charter by Waterfront Shipping, a subsidiary of Methanex, one of the largest methanol suppliers. 

Glow plug concept

An alternative to pilot fuel is to install a glow plug that initiates the combustion. Indications suggest that methanol has god surface ignition properties and that it could be a feasible alternative. Especially for applications where a separate pilot fuel systems is not practical and dual fuel operations is not an advantage or requirement.

The glow plug would also need to be suited for the high pressure and temperature environment in the cylinder but modern glow plugs should be able to withstand the demands with good enough reliability and lifetime. For good efficiency, the glow plug and fuel injector nozzle geometry is important to control in order to achieve short ignition delay. 

Different alternatives for the installation of a glow plug are possible. For example installation directly inside the cylinder (as in Figure 7) or in a pre-chamber to the cylinder with higher fuel-air ratio.

During the right circumstances there might be enough heat in the cylinder to ignite methanol once the engine is heated. In that case the glow plug use might be limited to start-up and low load operation. 

 

Figure 7: An example of a glow plug installation. The combustion is initiated by the hot surface of the glow plug.

The absence of pilot fuel diffusion might result in lower than Tier III NOX emissions despite compression ignition.

Ignition improvers

In addition to glow plugs or pilot fuel an ignition improver can be used to raise the cetane number of the fuel to promote ignition. Different ignition improvers are commercially available. However, little research has been done on how they work with methanol during the last decade as much attention has been focused on ethanol. Scania have gained extensive experience of using an ignition improver called Beraid in ethanol powered busses. 

The similarities between the two alcohols encourage some extrapolation of the results achieved. The Scania engines used for ethanol are diesel engines that have undergone some modifications. In addition to the fuel system that has been adopted for light viscosity fuel the compression ratio of the engines have been raised from 18:1 to 28:1. The fuel used is called Etamax D and consists of, in addition to ethanol, about 5 % ignition improver and 3 % denaturants and corrosion inhibitors. While the ignition improver makes it possible to run the engine according to the diesel cycle and to get quite high efficiency a possible disadvantage is the high cost of the ignition improver. As substantial modifications are still needed to the engine, it is questionable if this is the preferred route for all applications. Still, during the right circumstances it might be a plausible route for particularly smaller engines as an alternative to the use of glow plugs. 

Direct injection Otto cycle combustion

The direct injection Otto cycle concept relies on virtually the same hardware setup as the high pressure Diesel concept with the difference being the time of fuel injection. If methanol is injected to the cylinder during the early part of compression a premixed flame front could be achieved. The combustion is initiated by a burst of pilot diesel at TDC. NOx emissions are likely to be lower but emissions of formaldehyde, THC and CO is likely to be higher compared to diesel combustion. 

Concerns related to all premixed combustion especially during high load and load transients are limitations due to knocking and pre-ignition which would necessitate lower compression ratio and thus a larger bore. Expensive modifications that would also result in lower efficiency when using only diesel fuel compared to an unmodified engine. 

Figure 8: Direct injection Otto cycle: 1) Methanol is injected to the cylinder during the early part of combustion. 2) The air-methanol mixture is compressed by the cylinder. 3) A burst of pilot diesel is injected at TDC to ignite the air-methanol mixture. 4) The high pressure exhaust gases expand and drive the crankshaft.

Material incompatibilities are also introduced as the cylinder liner is exposed to methanol as well as possible methanol content in the blow-by-gases. Concerns are also related to the limited time for the methanol to achieve a homogenous mixture with the air before combustion. 

Low pressure Otto cycle combustion

The low pressure Otto cycle differs from the high pressure in particular with regards to the fuel injection timing. Several concepts are possible. 

Port injection 4 stroke “dual fuel”

Dual Fuel engines have been developed to allow engines to operate on gas as an alternative to oil. During operation on oil the engine runs as a normal diesel engine. During operation on gas the engine instead use the Otto cycle. Gas is injected through a gas injector, normally located in the cylinder head upstream from the inlet valves, and thus mixed with air in the cylinder and inlet channel. At TDC a burst of pilot diesel initiate the combustion. 

Instead of a gas valve, a methanol injector could be used. Both high pressure valves that atomises the fuel and vaporised methanol have been suggested.

Dual fuel engines normally operate on lean mixtures in gas mode as the pilot fuel is sufficient to ensure combustion. This allows for higher compression ratio and thus higher efficiency. 

On medium and high load throttling is usually not necessary; Wärtsilä uses a waste gate in the turbine to lower the charge air pressure during medium load to improve the fuel/air ratio but a throttle might be necessary at low load. 

Figure 9: Premixed Otto combustion: 1) Methanol is mixed with air during the intake period. 2) The air-methanol mixture is compressed in the cylinder. 3) A pilot injection of diesel starts the combustion at TDC.

As with direct injection knocking is a concern, for a retrofit the compression ratio would likely need to be lowered by enlarging the bore. Material incompatibilities concerns and emission levels are similar to other Otto concepts. 

Low pressure 2 stroke dual fuel

Experience of dual fuel engines have previously mostly been related to four stroke engines but have also been developed for larger two stroke engines. The Wärtsilä 50 RT-flex DF relies on low pressure fuel injection to the cylinder during the early part of compression. This allows for relatively low injection pressures (<10 bar) and consequently a less complex and a less expensive fuel injection system. 

As the fuel injection occurs after the air inlet ports are closed by the cylinder the air supply should not be affected by the corrosive properties of methanol. A potential problem is that it might be hard to achieve a homogeneous fuel/air mixture in the cylinder before ignition. 

 

Figure 10: 2-stroke Otto combustion: 1) Scavenging, air inlet in bottom of cylinder is open and exhausts are vented through the open top valve. 2) Methanol is injected to the cylinder during the early parts of combustion. 3) The methanol-air mixture in the cylinder is ignited by a pilot injection of diesel at TDC.  

As with similar technologies for four stroke engines, the fuel mixture in the cylinder is ignited by an injection of diesel at TDC. According to Wärtsilä, the amount of pilot diesel is significantly lower compared to the high pressure diesel concept. At full load the amount of pilot diesel is about 1 % of the total fuel energy and stays constant through the load range. Knocking is prevented by lowering the maximal output from the engine by 15-20 %. 

When operating on LNG emissions of NOX meet Tier III requirements and methanol operation is likely to meet it as well. 

Spark ignited

Both Rolls Royce and Wärtsilä have developed spark ignited gas engines utilizing basically the same principle. Gas is mixed with air before the inlet valves and fed to the cylinder. A portion of the mixture with higher gas concentration is fed to a pre chamber where a spark initiates the combustion at the end of compression. The flame from the pre chamber in turn ignites the fuel mixture in the cylinder. The advantage with the spark plug route is that no other fuel is needed to initiate the combustion but it also limit the flexibility as dual fuel operation is not supported. 

Compared to a duel fuel engine the compression ratio is lower to prevent knocking concern regardless of load and power transients. Combustion temperatures are kept low with a lean fuel/air mixture and emissions are comparatively low. The current engines are pure gas engines but conversion to methanol should be possible. Rolls Royce claim that their gas engine relying on spark plugs achieve higher efficiency and better load response compared to use of pilot diesel to initiate the combustion. 

 

Figure 11: Principal sketch of a spark ignited gas engine. The spark plug ignites the rich fuel mixture in the pre-chamber that in turn ignites the leaner fuel mixture in the cylinder.

As the spark plug is located in a pre chamber it is partially protected from the high pressure in the cylinder. This allows spark plugs to be used in high compression engines with good enough reliability.

Combined combustion cycle

In addition to operate the engine according to either the diesel or the Otto principle, the possibility to use both is possible. This can be done in a lot of different ways. Broadly either by using different methods during different running conditions by using both to a larger or smaller extent at the same time. 

Mixed combustion cycle

A suggested method to get both the benefit of low NOX emissions and good load response and high efficiency is to run the engine according to the Otto principle at low load and according to the diesel cycle at high load. Both modes rely on an injection of pilot diesel to initiate the combustion. 

At low load the engine operates according to the premixed Otto cycle with pilot diesel to start combustion at TDC. 

At high load the injection of methanol is delayed so that it occurs after the pilot fuel and thereby switching from Otto to Diesel mode. 

This allows for low NOX formation at low load combine with avoidance of knocking problems at high load. As there is no need to lower the compression ratio the efficiency should over all be better and it is still possible to operate the engine on diesel fuel as backup with no loss of performance in relation to a standard engine. 

Homogeneous Charge Compression Ignition

One method to do this is a concept called Homogeneous Charge Compression Ignition (HCCI). Like an Otto engine fuel and air is mixed inside the cylinder during compression, the difference is that HCCI like a diesel engine relies on spontaneous combustion of the mix due to the higher temperature caused by compression. As combustion simultaneously starts at several locations in the cylinder combustion is nearly instantaneous. The concept has shown good results with regard to efficiency and NOX and soot emissions, although hydrocarbon emissions have shown to be somewhat higher than the normal Otto engine. The difficult part is to control the ignition, this is easy in a normal Otto engine by controlling the spark plug or in a diesel engine by injecting fuel but as HCCI relies on spontaneous combustion of a premixed fuel it becomes much harder. By better understanding of the combustion process in combination with advanced engine control units it is possible to achieve results by varying parameters like fuel temperature, cylinder pressure and air quality. 

The air quality is controlled by Exhaust Gas Recycling (EGR). By using EGR the oxygen content is lowered and as a result the combustion temperature. This lowers NOX formation but side effects are more soot and lower efficiency.

Partially premixed combustion

One particular adoption of HCCI is called Partial Premixed Combustion (PPC) and is the focus of research by Bengt Johansson at LTH in Lund. To achieve better control of the combustion and at the same time high efficiency and low emissions the HCCI concept in combination of direct injection close to TDC is used. 

The fuel injection basically occurs in two steps: 

  1. About 60 % of the fuel at -60° TDC to create homogeneous mixture
  2. Rest of fuel about TDC to initiate combustion

Depending on parameters such as fuel type, compression ratio and amount of EGR the combustion might progress quite differently. An example of how cylinder pressure and rate of heat release relate to crank angle is shown in Figure 12. 

Figure 12: An example of average cylinder pressure and rate of heat release to crank angle (0 is TDC) during a PPC experiment using gasoline as fuel. The figure shows how fuel is injected in two bursts at different crank angles. 

Studies at LTH have showed good potential for the use of low cetane fuel such as gasoline, the longer ignition delay compared to diesel fuel lowers the risk of violent combustion and allows for higher compression ratio and less use of EGR that lowers the combustion efficiency. 

To control the combustion and to achieve high efficiency over a wide power band is still a challenge. PPC as well as other HCCI concepts is still in the research phase and is not ready for large scale implementation but could prove to be a feasible development path towards more efficient internal combustion engines. Experiments have been performed with ethanol that have showed potential in the PPC process and it is reasonable to believe that methanol could also work well.