YANMAR Technical Review

Abstract

Alternative fuels are becoming increasingly popular in the industrial engine industry with the aim of achieving a low-carbon society.
Paraffinic alternative fuels such as hydrotreated vegetable oil (HVO) and gas-to-liquid (GtL) are attracting attention as alternative fuels that are expected to significantly reduce greenhouse gas emissions over the entire fuel life cycle.
The greatest advantage of these fuels is that they have a molecular structure similar to that of conventional diesel and can be used as drop-in fuels, thus accelerating Yanmar’s efforts to create a low-carbon society without compromising the product appeal of the industrial engines it has cultivated over the years. Against this background, Yanmar has conducted demonstration tests on the applicability and effectiveness of paraffinic alternative fuels.

1. Introduction

It is anticipated that industrial engines will undergo an energy transition away from petroleum fuels as part of the shift to a low-carbon society. Paraffinic alternative fuels can be produced by hydrotreatment and synthesized from feedstocks such as natural gas, vegetable oils, and animal fats. Their use can reduce life-cycle CO₂ emissions by up to 90% compared to conventional diesel refined from petroleum. Work done by Yanmar to verify the use of paraffinic fuels as a diesel alternative in its small industrial diesel engines has demonstrated that this can be done with no changes to existing engine specifications.

2. Different Types of Liquid-Fuel Diesel Alternatives and Their Characteristics

2.1. Fatty Acid Methyl Ester (FAME)

The biodiesel fuels produced by conventional techniques use renewable resources such as soya bean, rape seed, palm, or other vegetable oils or animal fats as a feedstock and have FAMEs produced by transesterification as their main component. While such fuels help to reduce CO₂ emissions, their main use is for blending with conventional diesel as they suffer from quality issues, including clogging of filters and a tendency to degrade or oxidize plastics.

2.2. Hydrotreated Vegetable Oil (HVO)

HVO is a paraffinic alternative fuel that, while derived from the same feedstocks as FAME biodiesel, is made by using hydrotreatment to remove oxygen instead of by transesterification. As this results in a molecular system similar to conventional diesel, it can be used in 100% concentration without blending and therefore should result in much greater CO₂ emission reductions than FAME biodiesel blends. Due to these advantages, it is already available on the market, primarily in Europe, and further widespread growth in its use is anticipated.

2.3. Gas-to-Liquid (GTL)

GTL is synthesized from natural gas and, like HVO, is classified as a diesel-equivalent paraffinic alternative fuel. While the different feedstock and production process for GTL mean it offers smaller benefits for CO₂ reduction than FAME biodiesel or HVO, its CO₂ emissions are lower than those for conventional diesel and it has the potential to receive greater attention in the future as a means of reducing greenhouse gas emissions because of its use of methane as a feedstock.

3. Technical Trials for Use in Industrial Engines

Yanmar has undertaken technical trials using both HVO and GTL. While the feedstocks and production processes for these two paraffinic alternative fuels are different, they have the same molecular structure and fuel characteristics.

3.1. Excellent Combustion Performance

Loss of engine output is a concern with paraffinic alternative fuels because they tend to have a volumetric calorific value that is about 5% less than that of conventional diesel. However, because the volumetric flow rate of a fluid is theoretically inversely proportional to its density, the lower density of paraffinic alternative fuels means they can also be expected to have a higher flow rate. Yanmar assessed how these characteristics of paraffinic alternative fuels change output performance relative to conventional diesel, as shown in Fig. 2. The left of the figure shows how output performance is very similar to that for conventional diesel when used in an engine with a common rail system. The graph on the right shows the volumetric fuel flow when the engine is running, suggesting how the higher flow rate for the paraffinic alternative fuel compensates for its lower calorific value and leaves the output characteristics unchanged. On the other hand, a loss of output compared to conventional diesel does sometimes occur on engines that use mechanical fuel injection.

Fig. 3 shows the combustion characteristics of paraffinic alternative fuels and their impact on exhaust emissions performance. The figure on the left shows a graph of cylinder pressure and heat release rate during combustion. This shows that the ignition delay time (the time from fuel injection until the start of combustion) tends to be shorter for paraffinic alternative fuels as these have a higher cetane number than conventional diesel, cetane number being an indicator of ease-of-ignition. The graph on the right shows the NOx and PM emissions from a diesel engine running on conventional diesel and how these are both lower when using paraffinic alternative fuel. This is likely a result of less NOx being produced by high-temperature combustion due to the lower cylinder gas temperature and a lower proportion of premix combustion (combustion that occurs in the part of the diesel engine combustion process where fuel and air are mixed); a result of the shorter ignition delay time visible in the combustion characteristics. The reduction in particulate matter (PM) was likewise likely due to paraffinic alternative fuel having fewer aromatic components that generate PM.

3.2. Maintaining Reliability with No Changes to Existing Specifications

Wear or seizing of fuel injection components is a concern with paraffinic alternative fuels because they have low viscosity and provide less lubrication than conventional diesel. Fuel leaks are also a potential risk as the lower level of aromatic components means that plastic parts do not swell as much as they do with conventional diesel. Accordingly, Yanmar conducted engine durability testing to ensure that the new fuel could be used by customers without concern for these issues.
Fig. 4 shows the trend in fuel injection amount and engine output during engine durability testing. The top graph shows the fuel injection amount and the bottom graph shows the engine output. Steady performance is evident in both cases. Moreover, no sign of fuel leaking from plastic components was found during the engine durability testing. These results demonstrate that engine reliability can be maintained with no changes to existing specifications.

3.3. Improved Starting Performance in Cold Temperatures

As paraffinic alternative fuels have similar fluidity to winter diesel, it is suitable for use in cold conditions.
Fuel is typically difficult to ignite in the cold because the temperature inside the cylinders when starting the engine is too low, making engines hard to start and resulting in the emission of unburned fuel as white smoke. Anticipating that the excellent ignitability of paraffinic alternative fuels would help to improve performance, Yanmar tested engine starting under cold conditions.
Fig. 5 shows the results of these tests. The graph at the top shows the engine speed when starting. The speed ramps up more quickly for the paraffinic alternative fuel than it does for conventional diesel, demonstrating the improvement in starting performance. Similarly, the photographs in the bottom half of Fig. 5 show the amount of white smoke at 5 seconds after starting. The results demonstrated that white smoke emission ended more quickly for the paraffinic alternative fuel than for conventional diesel.

4. Conclusions

This article has described the results of testing to verify the use of paraffinic alternative fuels in Yanmar’s small industrial diesel engines. Paraffinic alternative fuels have a molecular structure similar to that of conventional diesel and testing demonstrated that they also have equal or better functional performance for the same engine specifications. This presents opportunities for accelerating action on achieving a low-carbon society without compromising the appeal of its diesel engines that Yanmar has cultivated over the years. In the future, Yanmar intends to continue its technology development efforts so that it can provide its customers with a wide range of options.