Dual-Fuel Diesel Engine Performance Evaluation

In the evolving landscape of automotive engineering, the pursuit of cleaner and more efficient power sources has led to significant advancements in internal combustion engines. One such innovation is the conversion of conventional diesel engines to operate on a dual-fuel system, primarily utilising natural gas (NG) with a small amount of liquid diesel fuel acting as an ignition source. This mode, often referred to as a “Pilot Ignited Natural Gas Diesel Engine – PINGDE”, presents a fascinating area of study for its unique combustion characteristics, performance metrics, and emission profiles. While the core principle of a diesel engine involves compression ignition, adapting it to burn gaseous fuels introduces a complex interplay of physical and chemical processes that fundamentally alter its operational dynamics. This article delves into the evaluation of such direct injection (DI) dual-fuel diesel engines, drawing upon recent research to highlight key performance indicators and emission characteristics, offering a comprehensive overview for enthusiasts and professionals alike.

Understanding the Dual-Fuel Concept

The PINGDE operating mode is characterised by natural gas providing the majority of the engine's power output. Typically, a small 'pilot' quantity of liquid diesel fuel, often around 10% of the total fuel supplied at full load (on an energy basis), is injected near the end of the compression stroke. This pilot injection serves a critical role: it acts as the ignition source for the primary gaseous fuel-air mixture. Engine power output is then precisely controlled by varying the amount of the primary gaseous fuel, while the pilot diesel fuel quantity is generally kept constant.

Natural Gas Introduction Methods

Natural gas can be introduced into the engine cylinder in a couple of ways. Most commonly, NG is inducted or injected into the intake manifold, where it mixes uniformly with air before entering the cylinder. This creates a homogeneous NG-air mixture that is then compressed, similar to conventional diesel operation. However, some advanced applications involve direct injection of NG into the cylinder shortly before the end of the compression stroke. This latter technique holds the promise of better fuel economy and more efficient combustion, while also helping to maintain the power output and thermal efficiency comparable to an equivalently-sized conventional diesel engine. The trade-off, however, is the necessity for developing specialised high-pressure gaseous injectors.

The Role of Pilot Diesel Fuel

Regardless of how natural gas is introduced, the pilot diesel fuel is invariably injected into the cylinder via the conventional diesel fuel injection system. The fundamental reason for this lies in the distinct ignition properties of natural gas compared to diesel. Ignition processes are commonly related to octane number (for spark-ignition engines) and cetane number (for compression-ignition engines). Methane, the primary component of natural gas, possesses a high octane number but a low cetane number. This means that, unlike diesel, natural gas does not spontaneously ignite under the typical compression ratios and temperatures found in conventional compression ignition (CI) engines. It requires a controlled ignition source, which is precisely what the pilot diesel fuel provides. The injection of this small amount of diesel into the hot, compressed environment leads to the ignition of the diesel droplets, which in turn ignites the surrounding gaseous fuel-air mixture, allowing the flame to propagate throughout the combustion chamber.

Combustion and Performance Characteristics

Evaluating the performance of dual-fuel engines involves scrutinising various parameters that define their operational efficiency and power delivery. Research into PINGDEs has consistently focused on combustion and emissions characteristics, alongside critical operational variables like pilot fuel size, injection timing, and fuel composition.

In-Cylinder Pressure Dynamics

Studies have shown varied effects on in-cylinder pressure in dual-fuel operation compared to conventional diesel operation (CDO). Some research indicates that dual-fuel operation (DFO) results in lower peak cylinder pressure. This is often attributed to the higher specific heat capacity of the natural gas-air mixture during the compression stroke, and a later ignition with a slower combustion rate at the early stages. The prolonged ignition delay in DFO means that combustion pressure is moderated as the piston moves away from Top Dead Centre (TDC) during the expansion stroke. At low loads, the combustion duration in DFO tends to be longer due to slower burning rates, but at high loads, it can become shorter, likely due to improved mixture strength as gaseous fuel increases.

Conversely, other studies have reported higher peak in-cylinder pressures with DFO at high loads, potentially due to the use of a smaller pilot fuel quantity which leads to a very fast burning rate of the large amount of natural gas once combustion commences. This highlights the sensitivity of combustion dynamics to pilot fuel quantity and engine load.

Heat Release Rate (HRR)

The Heat Release Rate (HRR) provides crucial insights into the combustion process. In conventional diesel combustion, the HRR curve typically exhibits a two-peak pattern, representing the premixed and mixing-controlled combustion phases. In dual-fuel engines, the combustion process can also be divided into distinct phases: ignition delay of the diesel pilot, premixed diesel combustion combined with gaseous fuel ignition, fast burning of the gaseous fuel, and late combustion. While some studies show that DFO maintains a dual-peak HRR pattern, particularly when diesel fuel contributes significantly to the total energy, others report a single-peak pattern, indicating a very fast burning rate of the gaseous fuel, especially at high loads where over 90% of the fuel can be consumed quickly. A longer ignition delay is consistently observed in dual-fuel mode due to the introduction of gaseous fuel, which alters both the physical and chemical processes during this period.

Brake Specific Fuel Consumption (BSFC) and Thermal Efficiency

One of the most critical performance metrics is Brake Specific Fuel Consumption (BSFC), which measures how efficiently an engine converts fuel into useful work. Research generally indicates that total BSFC is higher under DFO compared to CDO at the same operating conditions, particularly at low loads. This is often attributed to the poor utilisation of the gaseous fuel, mainly due to lower temperatures and suboptimal fuel-air ratios inside the combustion chamber. However, at high loads, the BSFC values for DFO tend to converge towards those of CDO, though often remaining slightly higher, as gaseous fuel utilisation improves.

A significant point to note in BSFC comparisons is the need for correction based on the Lower Heating Value (LHV) of the fuels. Without this correction, comparing BSFC directly between natural gas and diesel can be misleading, as their energy content per unit mass differs considerably. When corrected, the total BSFC under DFO would typically be even higher, leading to a further reduction in brake thermal efficiency.

Here's a simplified comparison of BSFC based on conceptual findings:

Brake thermal efficiency typically suffers a reduction in DFO compared to CDO. This is primarily because the introduction of natural gas reduces the partial pressure of oxygen in the intake charge and lowers temperature and pressure levels at the end of the compression stroke due to the higher specific heat capacity of the mixture. Consequently, the ignition delay increases, and a larger portion of the combustion process occurs during the expansion stroke, reducing the useful power output. Poor gaseous fuel utilisation and slower burning rates at low loads further exacerbate this efficiency drop.

Exhaust Emissions Characteristics

The environmental impact of an engine is largely determined by its exhaust emissions. Dual-fuel operation presents a mixed bag of results in this regard, with some pollutants significantly reduced while others increase.

Nitrogen Oxides (NOx) Emissions

Generally, the use of natural gas in dual-fuel engines has a positive effect on NOx emissions, leading to lower concentrations compared to conventional diesel operation. At low loads, the reduction is slight and attributed to lower in-cylinder temperatures caused by the slower burning rate of the gaseous fuel. At high loads, a more considerable reduction is often observed due to less intense combustion, lower temperatures, and reduced oxygen concentration (as NG replaces an equal amount of air). However, some studies present contradictory findings, reporting higher NOx emissions at high loads in DFO. This can be attributed to elevated in-cylinder temperatures due to improved gaseous fuel combustion and potentially increased oxygen concentration if the NG, being a low-carbon fuel, requires less oxygen for complete oxidation, leaving more oxygen available for NOx formation.

Soot and Particulate Matter Emissions

One of the most significant environmental benefits of dual-fuel operation is the drastic reduction in soot emissions, regardless of engine operating conditions. Natural gas, composed mainly of methane, has a very small tendency to produce soot. This makes DFO a highly effective technique for minimising particulate matter. However, there are exceptions; some research has unexpectedly reported an increase in soot and particulate matter under DFO. This counter-intuitive result has been linked to increased impingement of liquid fuel on in-cylinder surfaces. As natural gas induction can decrease cylinder pressure, it reduces resistance to the diesel fuel spray, leading to increased spray tip penetration and potentially more wall impingement and subsequent soot formation.

Carbon Monoxide (CO) and Hydrocarbon (HC) Emissions

Conversely, CO and HC emissions are generally considerably higher in dual-fuel operation relative to CDO. This is particularly pronounced at part load, where lower charge temperatures and fuel-air ratios lead to slower burning, allowing quantities of fuel to escape the combustion process unburnt. While an improvement in fuel utilisation and increased burned gas temperature at high loads can reduce these emissions, especially at lower engine speeds where there is more time for combustion, CO and HC levels typically remain much higher than in conventional diesel operation. The reported CO and HC emission traces often reflect the quality of the combustion process, with higher levels indicating less efficient burning. It's also noted that representing emissions as parts per million (ppm) or percentages can be misleading; using an Emission Index (EI) is often preferred as it unambiguously expresses the amount of pollutant formed per unit mass of fuel, independent of dilution or combustion efficiency.

Carbon Dioxide (CO2) Emissions

Unsurprisingly, CO2 emissions are typically reduced under dual-fuel operation compared to conventional diesel. This is a direct consequence of natural gas having a lower carbon-to-hydrogen (C/H) ratio than diesel fuel. This confirms that dual-fuelling diesel engines with NG is an effective strategy for reducing a prominent greenhouse gas, especially at high engine loads.

Operational Limits and Factors Affecting Performance

Beyond the core performance and emissions, several operational aspects influence the effectiveness and feasibility of dual-fuel conversion.

Maximum NG Substitution

A crucial aspect of dual-fuel operation is determining the maximum possible natural gas substitution for diesel fuel. This limit is often dictated by the occurrence of severe diesel knock or end-gas auto-ignition, on one hand, or combustion loss (misfire, inability to sustain steady torque output), on the other. Studies have shown that stable engine operation can be maintained with high NG energy substitution levels, sometimes as much as 86%. However, these limits are highly dependent on engine speed and load. For instance, high NG substitution levels might be achievable across the entire load range at low engine speeds, but at higher speeds, the substitution level may need to be reduced significantly to avoid severe engine knock. This is because increased turbulence at higher speeds accelerates the combustion process, and with already rich NG mixtures at high loads, this can push the engine into knock conditions.

Influence of Injection Timing

The timing of fuel injection plays a critical role. In some converted dual-fuel engines, the engine control module may automatically advance injection timing in response to reduced diesel fuel flow when natural gas is introduced. While an advanced injection timing leads to an earlier start of combustion, the net effect of NG admission can still be a prolonged ignition delay. Understanding this interplay is vital for optimising performance, as injection timing directly impacts in-cylinder pressure and combustion efficiency.

Crevice Volumes and Emissions

An interesting point often overlooked in dual-fuel studies is the impact of engine design. Diesel engines typically have relatively large crevice volumes (e.g., the space between the piston side and cylinder wall, or above the top ring). When a diesel engine is converted to dual-fuel operation, these larger crevice volumes can trap a significant amount of unburned mixture, contributing to higher CO and HC emissions. This mechanical characteristic, inherent to the original diesel design, can become a notable challenge in dual-fuel applications.

Comparative Analysis: CDO vs. DFO

To summarise the complex interplay of factors, here's a general comparison of key performance and emission characteristics between Conventional Diesel Operation (CDO) and Dual-Fuel Operation (DFO):

Frequently Asked Questions (FAQs)

Why is pilot diesel fuel essential in natural gas dual-fuel diesel engines?

Natural gas (primarily methane) has a very low cetane number, meaning it does not spontaneously ignite under the typical compression ratios and temperatures found in conventional diesel engines. The small pilot injection of diesel fuel, with its high cetane number, provides the necessary ignition source for the natural gas-air mixture to combust effectively.

How does dual-fuel operation affect the peak pressure inside the engine cylinder?

The effect can vary. Generally, dual-fuel operation can lead to lower peak cylinder pressures compared to conventional diesel, especially at low loads. This is often due to the higher specific heat capacity of the natural gas-air mixture and a prolonged ignition delay. However, some studies show higher peak pressures at high loads, particularly with smaller pilot fuel quantities, due to very fast gaseous fuel burning rates.

Why is Brake Specific Fuel Consumption (BSFC) often higher in dual-fuel mode?

BSFC is frequently higher in dual-fuel operation, particularly at low loads, due to less efficient utilisation of the gaseous fuel. This can be attributed to lower combustion chamber temperatures and sub-optimal fuel-air ratios, leading to incomplete combustion. While BSFC improves at higher loads, it often remains slightly higher than in conventional diesel operation, especially when corrected for the differing Lower Heating Values (LHVs) of natural gas and diesel.

What are the primary emission benefits and drawbacks of dual-fuel diesel engines?

The main benefits include a significant reduction in NOx emissions (due to lower combustion temperatures and oxygen concentration) and a drastic reduction in soot and particulate matter (as natural gas produces very little soot). The main drawbacks are considerably higher CO and HC emissions, particularly at part loads, due to incomplete combustion and the presence of unburned fuel trapped in engine crevices. CO2 emissions are typically reduced due to natural gas's lower carbon-to-hydrogen ratio.

Can any diesel engine be easily converted to dual-fuel operation?

While many direct injection diesel engines can be modified for dual-fuel operation, it's not always a straightforward process. It often requires specific modifications to the fuel system (e.g., for natural gas induction or direct injection), and the engine's control system needs to be recalibrated to manage the dual-fuel strategy. Factors like the engine's original crevice volumes can also impact the success and emission profile of the conversion.

Conclusion

The evaluation of direct injection dual-fuel diesel engines, specifically those operating in the PINGDE mode, reveals a complex but promising pathway towards more sustainable power. While these engines offer compelling advantages in reducing harmful emissions like NOx and soot, they often face challenges in maintaining the same level of fuel efficiency and controlling unburnt hydrocarbon and carbon monoxide emissions, particularly at lower loads. The delicate balance between pilot fuel quantity, injection timing, and natural gas substitution levels critically influences performance, combustion characteristics, and emission profiles. Future research will undoubtedly focus on optimising these parameters, developing advanced injection technologies, and refining control strategies to unlock the full potential of natural gas in heavy-duty applications. Understanding these intricate dynamics is paramount for engineers and technicians involved in the development, maintenance, and operation of these advanced propulsion systems, paving the way for cleaner and more efficient automotive solutions in the future.

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