Exploring the Efficacy of Port Water Injection on Air–Fuel Ratios and NOx Emissions in Diesel Engines: An Experimental Study ^†

Abstract

Diesel engines are popular due to their efficiency, power, and fewer carbon emission features. But NOx emissions pose a significant challenge to diesel engines’ use. NOx emissions are the largest constituents of diesel engines’ exhaust pollutants, with proven adverse effects on environment and human health. Different emission control strategies have been in use to inhibit NOx emission in diesel engines and to satisfy the global environmental standards. Port water injection, a relatively new emission control technology offers a solution to effectively inhibit the NOx emissions without significantly changing the standard combustion mode of diesel engines. This study experimentally investigates the impact of port water addition on air–fuel ratios (lambda ratios) and NOx emissions in a high-speed diesel engine. This investigation is carried out through experimentation on the diesel engine connected on a test bench across four operating conditions representing one low, one medium, and two high loading conditions. The experimentation introduced multiple port water injection rates from 0 to 21 kg/h. Results showed a 3–8% reduction in the lambda ratio and a substantial 75–89% decrease in NOx emissions with water addition. Importantly, combustion remained in the standard lean mode, affirming the effectiveness of port water injection in curbing NOx emissions while maintaining the required air–fuel ratio (lambda ratio).

1. Introduction

Diesel engines are still in widespread use in the transport sector due to their high efficiency, high power, lean combustion, and low carbon emissions [1]. The overall composition of the diesel exhaust consists of only 1% harmful emissions which consist of carbon monoxide, hydrocarbons, particulate matter, and nitrogenous compound (NOx) emissions. Among these pollutants, NOx have the largest 50% share [2]. NOx emissions from diesel engines are dangerous for both the environment and public health. NOx emissions from diesel engines contribute to air pollution, smog formation, acid rain, and adverse health effects which include lungs and respiratory diseases such as bronchitis, pulmonary edema, bronchiolitis, and pneumonitis [3,4]. Considering the significant share and adverse effects of NOx emissions, various global legislations have been enforced to reduce NOx emissions from diesel engines in the transport sector. The Euro-6 and Australian ADR79/05 standards limit NOx emissions in passenger cars to 0.08 g/km. The Japanese New Post Long Term Standards set a stricter limit of 0.007 g/km. China 6B mandates a limit of 0.035 g/km, while the US Tier 3 Standards for car models in 2024 set a limit of 57 mg/mi. In India, the Bharat-VI standards impose a limit of 0.060 g/km. [5]. So, diesel engine and automobile manufacturers are implementing various emission control strategies such as fuel injection and valve timing control [6], exhaust gas recirculation (EGR) [7], and alternative fuels and fuel additives [8] to inhibit NOx emissions and meet emission standards.

Water injection is relatively a new technology for inhibiting NOx emissions in diesel engines. There are two types of water injection that can be implemented on a diesel engine. When water is directly added in the combustion chamber in a similar manner as the direct injection of fuel is done, then this type of water injection is referred to as direct injection. When water is not directly added in the combustion chamber, but rather it is added through the inlet port to be mixed with inlet air before entering into the combustion chamber in a similar manner as the indirect injection of fuel, then this type of water injection is referred to as port injection [9]. The conceptual illustration of water injection types is shown in Figure 1.

NOx emissions in a diesel engine are formed at high operating pressures and temperatures when nitrogen and oxygen in the air react during combustion. Port water injection adds the water to the incoming air in the inlet port. The added water in the incoming air absorbs combustion heat inside the engine cylinder, is vaporized, and reduces peak pressures and temperatures [11]. This pressure and temperature drop inhibits the formation of NOx, making water injection an effective method for reducing these harmful emissions. Additionally, the presence of water vapor dilutes the oxygen concentration, slowing down the combustion process and further reducing NOx production.

Numerous investigations consistently highlight water injection’s ability to significantly reduce NOx emissions in diverse engine types and operating conditions [12]. Available research is focused on studying the interplay between water injection, thermodynamic processes, and NOx emissions in diesel engines only. The relationship between water injection, air–fuel ratios (AFRs), and NOx emissions has not been studied yet.

The lambda ratio (λ) represented in Equation (1) compares the actual air–fuel ratio (AFR) to stoichiometric AFR (AFRS). AFRS represents the chemically balanced air and diesel ratio for combustion. Equation (2) illustrates diesel fuel’s stoichiometric reaction with C, H, O, and N representing carbon, hydrogen, oxygen, and nitrogen, respectively, and Equation (3) illustrates the lean combustion equation [13].

$$\mathsf{\lambda }={\displaystyle \frac{AFR}{\left(AFRS\right)}}$$

(1)

$$C_i{H}_j+\left(i+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}\right)\left(O_2+3.7N_2\right)=iC{O}_2+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{H}_{2}O+3.7\left(i+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}\right)N_2$$

(2)

$$C_i{H}_j+n\left[\left(i+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}\right)\left(O_2+3.7N_2\right)\right]=n\left[iC{O}_2+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{H}_{2}O+3.7\left(i+{\displaystyle \raisebox{1ex}{$j$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}\right)N_2\right]$$

(3)

where i represents the number of oxygen atoms, j represents the number of hydrogen atoms and n represents the moles of air. It is to be noted that n > 1 represents an excess of air rather than stoichiometric balance.

Ideally, λ = 1 signifies stoichiometric combustion, where the number of air and diesel moles align with Equation (2) [14]. When λ > 1, there is an excess of air moles compared to Equation (2), representing lean combustion, while λ < 1 suggests a deficiency of air moles relative to this equation, indicating rich combustion. Lean combustion reduces fuel consumption, whereas rich combustion increases it [15]. Despite its efficiency, lean combustion promotes NOx formation due to elevated pressure and temperature, leading oxygen, and nitrogen to react and form NOx compounds. This experimental study has the novelty of concurrently investigating the impact of port water injection on air–fuel ratios (lambda ratios) and NOx emissions across diverse engine conditions and establishing the efficacy of this emission control technology in terms of the sustenance of the lean combustion mode of the diesel engine.

2. Methodology and Experimentation

2.1. Engine Testbed

This research employed a 4-cylinder, 2 L, high-speed automobile diesel engine mounted on a test bench equipped with an eddy current type dynamometer. The studied diesel engine and associated test bench have been used in multiple studies previously [16]. The compression ratio, stroke, and bore dimensions were 18, 88 mm, and 85 mm, respectively. The rated maximum power and torque were 100 kW at 4000 rpm and 320 Nm at 1750 rpm, respectively. Port water injection was introduced into the diesel engine, allowing for variable injection rates. The experimental setup is shown in Figure 2.

2.2. Instrumentation

During experimentation, fuel flow, air flow, and water flow rates were measured using a positive displacement flow measuring system, a hot-wire anemometer, and a turbine flowmeter, respectively. NOx emissions were measured through a chemiluminescence measuring system. Taylor’s Error Propagation Theorem measured relative errors of 1.5% and 1.05% for NOx emissions and lambda ratio measurements, respectively.

2.3. Test Sets

This study conducted experiments on four distinct sets, each with different operating speeds and torque conditions. The combination of speed and torque conditions is the most suitable and practiced parameter to generate the varying loading conditions for a diesel engine when tested on a test bench. The selection of these sets is based on their sufficient representation of low, medium, and high load conditions according to the engine’s specifications. Water injection quantity was incrementally raised from zero to maximum using a randomized approach at each setting, with lambda ratios and NOx emissions measured accordingly at each setting at each respective water injection rate. At each setting, the water injection was stopped at the point where the diesel engine’s normal operation showed any signs of stalling. The engine operating parameters for each test set are detailed in

Table 1. Details of engine operating parameters at respective test sets.

Engine Parameters	Set 1	Set 2	Set 3	Set 4
Speed (rpm)	1512	1664	2059	1997
Torque (Nm)	44	114	155	202

, while the corresponding water injection rates are outlined in

Table 2. Details of water injection rate at respective test sets.

Sets	Water Injection Rate (kg/h)
Set 1		0				3				4				6				7				9
Set 2		0				3				5				7				9				11
Set 3	0		3			5			7			9	11			13		15			17	19
Set 4	0				3			5			7		9				13			17		21

.

3. Results and Discussion

Figure 3 shows the comparative results of the lambda ratio and NOx emissions, with the increase in water injection rate at set 1 representing low loading conditions. At the start of the experimentation, without the use of water injection, the lambda ratio was at 3.36, displaying lean combustion, while, at the same point, NOx emissions were at 37 g/h. The increase in the water injection rate to 6 kg/h. showed a negligible reduction in lambda ratio to 3.14, while NOx emissions dropped significantly down to 13 g/h. From 6 to 9 kg/h water injection rate, the drop in NOx emissions was not as significant as that observed in the previous interval. NOx emissions dropped from 13 to 10 g/h, while the lambda ratio decreased to 3.09. The overall percentage drop in the lambda ratio was 8%, remaining within lean combustion mode. The drop in NOx emissions was 75%. The results indicate that at low loading conditions, port water injection significantly decreases NOx emissions without altering the lean combustion mode of the diesel engine.

Figure 4 shows the comparative results of the lambda ratio and NOx emissions with respect to the increase in water injection rate at set 2, representing medium loading conditions. The values of the lambda ratio and NOx emissions without water injection were measured at 1.54 and 95 g/h. The graph indicates that initially, at a 3 kg/h water injection rate, the decrease in NOx emissions was very rapid and NOx emissions dropped down to 51 g/h. After that, a gradual decrease in NOx emissions was noted down to 23 g/h. for an increase in water injection rate up to 11 kg/h. During the entire increase in water injection, the lambda ratio gradually decreased and reached 1.50 at a water injection rate of 9 kg/h., and after that it remained unchanged. The results indicate that at medium loading conditions, port water injection reduced the lambda ratio by a negligible amount of 3% while a significant decrease of 76% in NOx emissions was noted in comparison to the lambda ratio. It is further noted that port water injection did not affect the lean combustion mode of the diesel engine at medium loading conditions.

Figure 5 shows the comparative results of the lambda ratio and NOx emissions with respect to the increase in water injection rate at set 3, representing high loading conditions in terms of elevated speed. The value of the lambda ratio without port water injection was measured at 1.42, while the value of NOx emissions was measured at 159 g/h. The increase in water injection rate up to 19 kg/h. caused a gradual decrease in NOx emissions which dropped to the point of 18.2 g/h. The study of the results further showed that the lambda ratio initially remained unchanged up to a 7 kg/h. water injection rate. Beyond that water injection rate, the lambda ratio gradually decreased to 1.34, corresponding to a 15 kg/h. water injection rate. The lambda ratio then remained constant throughout the experimentation. The overall percent decrease in the lambda ratio was noted to be only 6% while remaining within the lean combustion mode. In comparison, a large percent decrease of 89% in NOx emissions was noted.

Figure 6 shows the comparative results of the lambda ratio and NOx emissions with respect to the increase in water injection rate at set 4, representing high loading conditions in terms of elevated torque. The values of the lambda ratio and NOx emissions without port water injection were measured at 1.33 and 292 g/h. The water injection rate was increased up to 21 kg/h., which showed a gradual decrease in the values of NOx emissions. The decrease in NOx emissions continued down to 47 g/h. The lambda ratio decreased meagerly to 1.29, remaining within lean combustion mode. The overall percent change in the value of lambda ratio was only 3% while a significant decrease of 84% in NOx emissions was noted.

The results discussed above illustrate higher NOx emissions at elevated loading conditions regardless of water injection. This is due to the higher combustion pressure and temperature which were generated due to higher fuel consumption at elevated loading conditions. Additionally, the lambda ratio decreased with increased engine loading, unaffected by water injection. This happened due to the shift of combustion mode from lean to rich combustion as an effect of increased fuel demand by the diesel engine at elevated loading conditions.

Moreover, the increase in water injection consistently reduced both the lambda ratio and NOx emissions across all test settings and loading conditions. This decrease in lambda ratio can be attributed to the reduction in the number of air molecules which are replaced by water molecules. It is clear that water molecules do not chemically take part in stoichiometric or lean combustion equations. It is also proved that a lean combustion mode (λ > 1) persisted across all water injection rates, maintaining standard diesel engine operation at all test settings and loading conditions. Although water injection led to a slight decrease in the lambda ratio, the substantial reduction in NOx emissions is attributed to water’s cooling effect, which lowers in-cylinder pressure and temperature, limiting NOx formation.

4. Conclusions

This experimental study aimed to investigate the impact of port water injection, an emission control strategy, on the air–fuel ratio (lambda ratio) and NOx emissions of a high-speed automotive diesel engine. The lambda ratio and NOx emissions are among the parameters that define the combustion mode of the diesel engine. Diesel engines are operated in the lean mode of combustion where the lambda ratio is kept greater than 1, but the lean mode of combustion gives rise to higher NOx emissions. This study conducted extensive experiments on four distinct test sets, each representative of low, medium, and high loading conditions of engine operation. Under this experimentation, port water injection at rates ranging from 0 to 21 kg/h. was introduced in the diesel engine covering all test sets. As a result, the lambda ratio showed a small decrease of 3 to 8% in comparison to a large decrease of 75 to 89% in NOx emissions while maintaining the lean combustion mode of diesel engine at the same time, affirming the efficacy of port water injection in mitigating NOx emissions without substantial alteration to required air–fuel ratios.