**5. Use of DME in diesel engines**

#### **5.1 Fuel properties and special fuel injection system**

A pressurized fuel injection system for DME is an essential requirement. Thus, the tank, fuel pumps, fuel piping and the fuel injector have to be kept under suitable pressure. Fuel pumps have to pressurize the fuel circuit to a pressure which is higher than the saturation vapor pressure of DME at the operating temperature. This will prevent the DME fuel to vaporize and cause cavitation in the fuel circuit before the fuel injection into the cylinder. Temperature of the fuel inside the fuel injector reaches to 80°C and the pressure of the fuel circuit after the fuel pumps has to be increased accordingly. A pressure higher than 30 bar is considered adequate for keeping the DME in liquified form even at higher temperatures encountered during operation of the engine. Feed-pumps will be able to pressurize the fuel to the required pressure.

ASTM standard range for viscosity of liquid fuels has a range of 1.39 to 4.2 cSt at 40°C whereas viscosity of DME is within 0.185 cSt and 0.23 cSt. Low viscosity of DME will result in leakages past clearances used for sealing like plungers and barrels, seals and gaskets and pump gears etc. Low lubricity will result in high wear and seizure of the moving parts in fuel injection system. Viscosity and lubricity enhancing additives are added to the DME to overcome these problems.

Bulk modulus of DME is less than diesel by an order of magnitude. This implies that DME is much more compressible than diesel. High compressibility of DME will result in delay in the injection timings. Injection lag will be higher as compared to diesel and therefore the ECU has to be programmed accordingly. The compression work of DME in the fuel pumps and injectors is much higher than diesel fuel and the parasitic power for pumping of fuel is higher. Large compressibility of DME also results in injection instability and this problem can be overcome by modifying the nozzle design and control of fuel temperature.

**Table 6** compares the chemical and physical properties of DME, diesel fuel and LPG (propane, butane). Diesel properties are compared to DME to understand the similarity between the two in compression-based ignition, whereas comparison of DME to LPG is required to know about the similarity in fuel handling of the two fuels.

**Table 6** shows that auto-ignition temperature of DME is lower than diesel at pressure higher than atmospheric. Also, the cetane number of DME is higher than that of diesel. Thus, DME fuel is suitable in CI engines and has high potential to replace diesel fuel. At the same time, boiling point of DME is close to propane below Zero °C and the handling, storage and distribution of DME is similar to LPG. World over, LPG is used as cooking fuel and also for transport, therefore there is adequate experience in handling and storage of LPG. Use of LPG storage and handling facilities for DME with modifications to the seals, gaskets and certain metallic parts can be done with lower efforts and cost.

DME is an oxygenated fuel with an oxygen percentage of nearly 35%. Higher Cetane number results in lower ignition delays and smaller pre-mixed combustion


#### **Table 6.**

*Comparison of properties of DME and diesel fuel [12, 15].*

phase, lower peak cylinder pressures and lower NOx formation. Absence of C-C bonds leads to sootless combustion. In the DME molecule each carbon atom is bound to three hydrogen atoms on one side and oxygen atom on the other. Bond energy of C-H is 414 kJ/mol and that of C-O bond is 359.0 kJ/mol. Higher C-H bond energy is responsible for shorter ignition delays and higher cetane number of DME.

LHV of DME is almost half that of diesel, therefore to obtain the same horsepower, flow rate of DME is about 1.7 times that of diesel. This means larger storage tanks for DME, higher diameter pipes and tubes for fuel flow, higher flow capacity of the DME pumps and the fuel injectors. Duration of Injection (DOI) of DME will be longer than diesel and the Start of Injection (SOI) has to be advanced accordingly. Lower boiling point of DME translates into faster vaporization of injected fuel in the combustion chamber. This along with lower critical temperature of DME results in superheated vapor in the combustion chamber, adequate air-fuel mixing is ensured. Large heat of vaporization also lowers the in-cylinder temperatures and lower NOx emissions.

Chain combustion reaction is possibly through one of the following competing pathways [12]:

a. C–O bond fission (pyrolysis mechanism):

$$\text{CH}\_3\text{OCH}\_3 = \text{CH}\_3\text{O} + \text{CH}\_3.\tag{13}$$

b. Hydrogen abstraction (oxidation mechanism):

$$\text{4CH}\_3\text{OCH}\_3 + \text{O}\_2 = \text{4CH}\_3\text{OCH}\_2 + \text{2H}\_2\text{O}.\tag{14}$$

$$\text{CH}\_3\text{OCH}\_2 = \text{CH}\_2\text{O} + \text{CH}\_3 \tag{15}$$

As the C-O bond energy is smaller than C-H bond, distortion of the C-O bonds in the DME molecule weakens the bonding strength and the breakage of the C-O bonds earlier. Pyrolysis is more likely to start the chain reaction at relatively low temperatures, showing as lower auto-ignition temperature.

Change of piston and cylinder heads in the existing diesel engines may not be required other than design of the fuel injection nozzle to suit the volumetric flow rate and existing piston profile. However, in order to have an optimum design of combustion chamber to suit the spray characteristics of DME changes to the piston bowl and re-location of piston rings may be needed. For best performance of the engine valve timings may also need to be modified resulting in design and development of new camshafts. For modifying existing diesel engines, it becomes necessary to retrofit a new fuel injection system right from the fuel tank, feed pumps, pressure pumps, common rails and fuel injectors.

DME is similar to LPG in terms of safety. Vapor of DME is heavier than air and settles to the ground similar to LPG. Sufficient ventilation is necessary in locations where DME is being used whether for stationary installations or transport. Ignition limit of DME is 3.4% - 18.6% by volume and therefore necessary precautions have to be implemented which will be similar to LPG. Global Warming Potential (GWP) of a molecule is its adverse effect on climate change. GWP includes both the molecules lifetime and ability to absorb radiation. DME is atmospherically not dangerous and does not contribute to global warming.

Common rail fuel injection systems have been developed for DME fueled CI engines and these engines have demonstrated good engine performances and efficiency along with significant reduction in harmful exhaust emissions. This has been made possible by having a good control of the fuel injection characteristics and temperature. The common rail concept for DME fuel have also proven effective in simple and safe fuel handling. **Figure 9(a)** illustrate a comparison of the concepts use for DME fueled diesel engines, figure is self-explanatory. **Figure 9(b)** shows a schematic of the DME fuel storage and distribution system on engine.

**Figure 9(c)** presents a comparison of the simulated injection characteristics of diesel and DME digital hydraulic operating system (DHOS) injectors. Piston lifts in both the cases are similar, whereas the fuel injection pressure for diesel injector is about 1200 bar and for the DME injector it is around 800 bar. Duration of injection (DOI) for the DME injector is higher as compared to the diesel injector and similar trend is reflected in the period of needle lift of the injector. The injection rate of fuel for the DME case is higher than diesel all along the injection and the DOI is higher as can be seen in the bottom-most figure. Higher volume of fuel flow for DME vis-a-vis diesel can be seen in the figure due to lower density and LHV of DME compared to diesel.

#### **5.2 Emission characteristics**

Stoichiometric combustion of DME in air yields 1.91 m of CO2. This is equivalent to 66 gm of CO2 per MJ (LHV) of combusted DME.


Stoichiometric combustion of one gram of diesel *C*12*H*<sup>23</sup> yields 3.16 grams of CO2.


Combustion of one gm of DME in air emits less CO2 than combustion of one gm of Diesel, difference being 1.25 gm less CO2. Experimental investigations have brought out that there is substantial reduction in particulate matter (PM), NOx and combustion noise when DME is used as a fuel in CI engines. Combustion efficiency (BSFC) of DME fuel in a CI engine is similar to diesel (**Figure 10**) and so fuel consumption can be similar on an energy basis. Also shown in **Figure 10** are comparison of the road load emissions of NOx, CO, HC and Smoke (PM) on a DME and diesel fueled engine. Smoke is undetectable and NOx, CO, HC are lower in DME engine fitted with an oxidation catalyst.

**Figure 9.**

*Fuel injection system and injection characteristics of a DME fueled engine [16]. (a) Comparison of the fuel injection concepts for DME fueled engines. (b) DME storage, handling and injection system for a DME fueled diesel engine. (c) Comparison of simulated injection characteristics of Diesel and DME DHOS injector.*

*Replacement of Diesel Fuel by DME in Compression Ignition Engines: Case for India DOI: http://dx.doi.org/10.5772/intechopen.104969*

**Figure 10.** *Comparative analysis of emission data from neat DME & mineral fueled CI engine [17].*

#### **Figure 11.**

*NOx emission data from a DME-fueled 1.15 MW diesel power generation unit [18].*

**Figure 11** gives the NOx emission data vs. engine efficiency for a 1.15 MW diesel power generation unit engine fueled with DME and with different levels of EGR. As the DME fueled engine does not produce any soot and ultra-low PM emissions, higher EGR levels can be used on the engine to reduce the NOx emissions. Against a 950 ppm NOx regulation, as the EGR is increased, very low NOx levels of 30 ppm can be achieved albeit with an engine efficiency penalty of 3%.

#### **Figure 12.**

*Comparison of fuel consumption (BSFC), NOx and CO2 emissions on a 6-cylinder 7 liter turbocharge/intercooled heavy-duty diesel engine operating in Japanese D13 mode [19].*

**Figure 12** shows emission results on 6-cylinder 7 liter turbocharged/intercooled heavy-duty diesel engine operating in Japanese D13 mode driving cycle. NOx and CO2 emissions can be reduced with DME fueled engine vis-a-vis diesel engine at comparable fuel economy. In addition, combustion noise of DME fueled engines are lesser than their diesel counterparts.

In many countries research and development of DME fueled CI engines has been carried out and commercial trails done successfully. Japan, Europe, North America, China and South Korea are the leaders in development of DME fueled engines. A brief summary of the development of DME engines is presented in **Table 7**.
