The Influence of Powering a Compression Ignition Engine with HVO Fuel on the Specific Emissions of Selected Toxic Exhaust Components

Abstract

The aim of the research was to determine the potential of hydrotreated vegetable oil (HVO) in reducing nitrogen oxides and particulate matter emissions from the Perkins 854E-E34TA compression ignition engine. The concentrations of these toxic exhaust gas components were measured using the following analyzers: AVL CEB II (for NOx concentration measurement) and Horiba Mexa 1230 PM (for PM measurement). The measurements were carried out in the ESC test on a compression ignition engine with direct fuel injection and a turbocharger. The engine had a common rail fuel supply system and met the Stage IIIB/Tier 4 exhaust emission standard. Two fuels were used in the tests: diesel fuel (DF) and hydrotreated vegetable oil (HVO). As part of the experiment, the basic indicators of engine operation were also determined (torque, effective power, and fuel consumption) and selected parameters of the combustion process, such as the instantaneous pressure of the working medium in the combustion chamber, maximum pressures and temperatures in the combustion chamber, and the heat release rate (HRR), were calculated. The tests were carried out in accordance with the ESC test because the authors wanted to determine how the new generation HVO fuel, powering a modern combustion engine with a common rail fuel system, would perform in a stationary emission test. Based on the obtained research results, the authors concluded that HVO fuel can replace diesel fuel in diesel engines even without major modifications or changes in engine settings.

1. Introduction

Liquid alternative fuels are substances used to replace traditional fuels such as gasoline and diesel, usually to reduce greenhouse gas emissions and dependence on fossil fuels. Biofuels are often classified by generation, which refers to how the raw materials are produced and the production processes. There are typically three main generations of biofuels [1,2]. First-generation biofuels are produced from readily available plant feedstocks, such as corn, sugar cane, rapeseed, or palm oil. The production process for first-generation biofuels is relatively simple and usually involves fermentation or extraction of vegetable oil. However, their production can compete with food production, leading to land-use conflicts [3,4,5]. Second-generation biofuels are produced from non-food feedstocks, such as plant residues, stems, leaves, straw, or organic waste. Second-generation biofuel production processes are more technologically advanced and typically involve chemical, thermal, or biological processes, such as biomass thermochemical conversion or cellulose fermentation. Second-generation biofuels may have less impact on food production and may be more environmentally sustainable [6,7,8,9]. Third-generation biofuels include biofuels produced from algae, bacteria, or other microorganisms. These microorganisms can be grown in special reactors and used to produce biofuels such as biodiesel or biomethane. Third-generation biofuels have higher production potential than crops and can be produced on land unsuitable for traditional agriculture. However, third-generation biofuel production technologies are still in the development and commercialization phases [10,11,12].

As technology advances, more emphasis is being placed on developing second and third-generation biofuels due to their potential to reduce greenhouse gas emissions and have less impact on food production [1,13,14,15]. Such fuels include hydrotreated diesel. HVO fuel is a biofuel produced from vegetable oils or animal fats using hydrovet. This process involves subjecting the oil or fat to high temperatures and pressures in the presence of hydrogen and a catalyst, leading to hydrotreating, which removes oxygen from the oil or fat molecules [16,17,18,19]. HVO fuel is considered a clean fuel because its production eliminates impurities such as sulfur, nitrogen, and aromatic compounds in traditional diesel fuels [20,21]. As a result, HVO can potentially reduce emissions of atmospheric pollutants and greenhouse gases compared to conventional diesel. In addition, HVO fuel complies with diesel fuel quality standards, which means it can be used without modifying engines or infrastructure [22,23,24,25]. The main motivation for performing empirical tests on an engine dynamometer using HVO fuel was the search for alternative power sources for internal combustion engines. Because HVO fuel is often promoted as a more sustainable alternative to traditional fuels, the authors of this article decided to look more closely into the subject of this fuel. This was the first research of its kind using HVO fuel conducted at the Warsaw University of Technology. HVO fuel is becoming increasingly popular in many countries, including some in Europe, due to its environmental benefits and ability to replace traditional diesel fuel in various sectors, including road, marine, and air transport.

2. Materials and Methods

2.1. Fuel

During tests on a dynamometer bench at the Institute of Vehicles and Working Machinery, the engine was powered by two fuels. First, tests were carried out by powering the engine with diesel fuel as a basis for comparing the results obtained. Then, empirical tests were carried out by powering the engine with a second-generation biofuel, namely hydrotreated vegetable oil.

Table 1. Selected properties of HVO and diesel oil [26,27,28,29,30].

Properties	Unit	Method	HVO	Diesel Fuel
Kinematic viscosity	$\frac{{\mathrm{m}\mathrm{m}}^2}{\mathrm{s}}$	-	2.646	2.969
Density (at 15 °C)	$\frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}$	-	778	830
Dynamic viscosity	Pa·s	-	$2.06·{10}^{-3}$	$2.47·{10}^{-3}$
Cetane number	-	ASTM-D613	79.7	54.6
Pour point	K	ISO3016	215	234
Flash point	K	ISO2719	339	344
Cold filter Plugging point	K	EN 116	229	251
Total aromatic	% v/v	-	0	23.1
Polyaromatic	% v/v	-	0	3.0
Monoaromatic	% v/v	-	0.50	20.1
Flammability	K	-	334	347
Lower heating	$\frac{\mathrm{J}}{\mathrm{k}\mathrm{g}}$	-	$44.35·{10}^6$	$42.65·{10}^6$
Sulfur	% m/m	-	0.53	6.50
Carbon	% m/m	-	85	85.67
Oxygen	% m/m	-	0	0.61
Hydrogen	% m/m	-	15	13.72
Ash content	% m/m	EN ISO 6245	0.002	0.014
FAME	% v/v	-	0.05	5
Approx. formula	% v/v	-	C13H28	C13H24O0.06

shows the key physical and chemical parameters of both fuels. These properties alone give rise to the hypothesis that HVO fuel could replace diesel if its physicochemical parameters were similar to those of diesel listed in Table 1.

Hydrogenated vegetable oil fuel is a type of biofuel that is produced by hydrogenating vegetable oil to make a substitute for traditional petroleum fuels such as gasoline and diesel. The process is designed to change the chemical properties of vegetable oil to give it characteristics more similar to conventional fuel [8,27,31,32,33]. Hydrogenated fuel from vegetable oil can be used in internal combustion engines as an alternative to petroleum fuels. It is often considered an environmentally friendly option because vegetable oils are biodegradable and can be a renewable energy source, unlike petroleum fuels, which are a limited resource and generate greenhouse gas emissions [23,24,31,32,33]. An essential advantage of HVO fuel is its freezing point, which can be as high as 215 K [34,35]. HVO fuel has a density similar to conventional diesel fuel, making it potentially suitable for direct use in existing diesel engines without modification. HVO fuel tends to have a low kinematic viscosity, which may affect its easier splashing of injectors in diesel engines. The heating value of HVO fuel is similar to that of diesel, which means it can provide a similar amount of energy when burned. HVO fuel is typically sulfur-free and contains far fewer other pollutants than diesel fuel, which can help reduce harmful emissions into the atmosphere [36,37,38]. One solution that will consider the environmental protection goal, while not requiring rapid reorganization of the operations of companies and ordinary households, is to use biofuel as a substitute for diesel fuel [39,40]. Such a fuel is hydrotreated vegetable oil.

2.2. Test Stand

Empirical tests were conducted on an engine dynamometer. The bench was based on a 4-cylinder Perkins 854E-E34TA compression ignition engine. The test stand is equipped with a SCHENCK brake for measuring engine torque. The accuracy of the torque measurement is ±2 Nm. As part of the tests, it is also possible to measure fuel consumption with an accuracy of 1% and also to measure the engine’s crankshaft speed. In addition, AVL IndiSmart 612 (software version 2010) software was used to circle the engine cylinder pressure. The test stand and measuring apparatus are shown in Figure 1.

Technical data of the engine are shown in Table 1. Measurements of the toxic components of the exhaust gas were carried out on AVL CEB II and Horiba Mexa 1230 PM analyzers.

Table 2. Technical specifications of Perkins 854E-E34TA engine [41].

Mode	Unit	Engine Speed
Cylinder arrangement	-	in-line
Number of cylinders	-	4
Type of injection	-	direct
Compression ratio	-	17
Cylinder diameter	mm	99
Piston stroke	mm	110
Engine displacement	dm3	3.4
Engine type	-	compression ignition
Nominal power	kW	86
Nominal power speed	rpm	2200
Maximum torque	Nm	450
Speed at maximum torque	rpm	1400

shows the measurement ranges and measurement errors of toxic exhaust components on the AVL CEB II analyzer.

Table 3. The error associated with the AVL CEB II [42].

Species	Range	Analyzer Error
NOx	Low: 30–5000	±1 ppm
	High: 50–10,000	±2 ppm

shows the error associated in AVL CEB II.

Particulate matter (PM) was determined by a real-time continuous analyzer. Its measuring range was 0–300 mg/m³ (measurement accuracy was 1 mg/m³).

2.3. ESC Test

Exhaust emissions from internal combustion engines are dependent on engine operating states, both static and dynamic. Awareness of the dependence of the emission of toxic components from internal combustion engines on their operating states has led to the need to develop tests that correspond to actual engine operating conditions. This has led to the development of several static and dynamic tests typically used in approval procedures. The ESC (European Stationary Cycle) test is one of the more extensive static tests. ESC is a 13-phase, steady-state procedure, which has also been called the OICA or ACEA cycle [43]. Engine testing in the ESC test is a procedure that evaluates the emissions and fuel efficiency of engines. The test simulates urban driving conditions, where stopping and acceleration are common. During the test, the engine is run stationary and subjected to various load cycles that simulate typical city driving conditions [44]. The ESC test cycle was introduced along with the European Load Response (ELR) test and the European Transient Cycle (ETC) test as part of the Euro III emissions [45]. For this test, engine tests are conducted on an engine dynamometer at steady state. The engine must run for a specified time in each mode, reaching full speed and load changes within the first 20 s [46,47,48]. Circumferential torque must be maintained within ±2% of maximum torque, while the specified speed must be maintained within ±50 rpm. Emissions of toxic exhaust components are averaged over the cycle using weighting factors. Emissions results are expressed in g/kWh.

Table 4. ESC test modes [49].

Mode	Weight, %	Engine Speed	Load, %
1	15	Low idle	0
2	8	A	100
3	10	B	50
4	10	B	75
5	5	A	50
6	5	A	75
7	5	A	25
8	9	B	100
9	10	B	25
10	8	C	100
11	5	C	25
12	5	C	75
13	5	C	50

shows the test modes of the ESC test.

To determine the speeds A, B, and C, it is necessary to determine two speeds. The speed of the motor at 70% of maximum power must be specified. This speed is denoted as $n_{hi}$. The motor speed for 50% of maximum power is then determined. This speed is denoted as $n_{lo}$. This makes it possible to calculate motor speeds A, B, and C [45,46]:

$$\begin{gathered} A=n_{lo}+0.25(n_{hi}-n_{lo}) \\ B=n_{lo}+0.50(n_{hi}-n_{lo}) \\ C=n_{lo}+0.75(n_{hi}-n_{lo}) \end{gathered}$$

High average load factors and high exhaust gas temperatures characterize the ESC test. Figure 2 graphically shows the process of conducting an ESC test on a dynamometer bench at Warsaw University of Technology.

Due to the fact that the test stand is equipped with the AVL IndiSmart system, it is possible to determine the pressure in the Perkins engine cylinder. The relative error of measurement of the gas pressure in the combustion chamber of the Perkins engine is about δ = 0.25%. This makes it possible to calculate the amount and rate of relative heat release in the combustion process. The formula to calculate the heat release rate is as follows [50]:

$$HRR=\frac{\kappa }{\kappa -1}p\frac{dV}{d\phi }+\frac{1}{\kappa -1}V\frac{dp}{d\phi }\hspace{1em}[\frac{J}{CAD}]$$

(1)

where

$$\frac{c_p}{c_v}=\kappa$$

(2)

The temperature in the cylinder can be determined using the equations of state of a perfect gas. Another parameter that can be calculated is specific fuel consumption (SFC). This is a parameter that can be calculated from the hourly fuel consumption (HFC) measured during experimental testing and the effective power (EP) at a given engine operating point [51]

$$SFC=\frac{HFC}{EP}·1000[\frac{\mathrm{g}}{\mathrm{k}\mathrm{W}·\mathrm{h}}]$$

(3)

$$HFC=\frac{m}{t}[\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}]$$

(4)

where

t—fuel mass consumption time [h] and m—fuel mass [kg].

The usable power is calculated based on torque (T) and crankshaft angular velocity

$$(\mathsf{\omega }):EP=\frac{T·\mathsf{\omega }}{1000}\left[\mathrm{kW}\right]$$

(5)

where

ω—angular speed of the crankshaft [rad/s] and T—torque crankshaft torque [Nm].

Figure 3 shows a schematic diagram of the test stand.

3. Results

This section presents the torque and effective power curves (Figure 4). The tests shown in this drawing were performed at the maximum (volume) fuel dose and in the entire crankshaft rotation speed range of the tested engine. The figure below (Figure 5) shows the hourly fuel consumption and specific fuel consumption curves. In this case, the same operating conditions of the combustion engine also occurred.

The figure below (Figure 6) shows the curves of the instantaneous pressure and heat release rate (HRR) as well as the maximum values of pressure and temperature of the working medium. The heat release rate was determined from Equation (1), and the temperature values were obtained from the gas equation of state using the AVL Concerto program.

Figure 7, Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12 show the combustion parameters in combustion chamber determined in the ESC test for both of the tested fuels.

Figure 13, Figure 14, Figure 15, Figure 16 and Figure 17 show the concentrations of nitrogen oxides and particulate matter as well as the emissions of the above-mentioned toxic exhaust gas components determined in the ESC test for both of the tested fuels.

4. Discussion

• Speed characteristics at a full load:

  -

  Obtaining higher values of torque and effective power (entire range) for the maximum HVO dose at the maximum volumetric dose of HVO fuel (Figure 4). Max 4% increase.

  -

  Achieving lower specific and hourly fuel consumption (whole speed range) for DF—max 2% (Figure 5).

• The analysis of the results of the instantaneous pressure and temperature in the combustion chamber as a function of the crankshaft rotation angle shows the following for HVO fuel:

  -

  Earlier start of the combustion process by 2 degrees CAD (Figure 6) and, consequently, an increase in the maximum temperature in the combustion chamber by approximately 30 K and in the maximum pressure by 0.3 MPa for the maximum load and rotational speed of 1300 rpm (Figure 7 and Figure 8). At lower loads, the differences are smaller for both fuels, and at an engine load of 25%, no difference was observed.

  -

  Earlier start of the combustion process, resulting in an increase in the maximum temperature in the combustion chamber by approximately 20 K and in the maximum pressure by 0.2 MPa for the maximum load and rotational speed of 1600 rpm (Figure 9 and Figure 10). At lower loads, the differences are smaller for both fuels, and at an engine load of 25%, no difference was observed.

  -

  Earlier start of the combustion process, resulting in an increase in the maximum temperature in the combustion chamber by approximately 17 K and in the maximum pressure by 0.16 MPa for the maximum load and rotational speed of 1900 rpm (Figure 11 and Figure 12). At lower loads, the differences are smaller for both fuels, and at an engine load of 25%, no difference was observed.

• Emissions of harmful substances:
  The speed characteristics (Figure 13) show that NOx concentrations are higher for HVO fuel (by a maximum of 18%), and particulate matter is 50% lower than for DF.
  The load characteristics for 1300 rpm (Figure 14) show that NOx concentrations are higher for HVO fuel, but only at the maximum load. As the load decreases, the trend reverses and HVO fuel shows a maximum of 7% lower NOx concentration than DF. In the case of HVO fuel, PM emissions in the entire range of tested load characteristics show lower emissions by up to 75% (at a low engine load).
  On the load characteristics for 1600 rpm (Figure 15), NOx concentrations are higher for HVO fuel, but only at the maximum load. As the load decreases, the trend reverses and HVO shows a maximum of 17% lower NOx concentration than DF. In the case of HVO fuel, PM emissions in the entire tested load range are lower by up to 58% (at a low engine load).
  On the load curve at 1900 rpm (Figure 16), NOx concentrations are about 17% lower for HVO, but only at the maximum load. As the load decreases, both fuels show similar emissions (maximum 4% difference)
  For HVO, particulate matter in the entire tested range is characterized by lower emissions by up to 56% (at a low engine load).
• The specific NOx emissions determined in the ESC test for the Perkins 854E-E34TA engine are 2.3% lower for HVO. Specific PM emissions are 43% lower for HVO compared to DF.

5. Conclusions

The main reason for obtaining differences in the selected and tested combustion process indicators seems to be differences in the physicochemical properties of the tested fuels. The most important of these are fuel calorific value, fuel density, and cetane number. HVO is a fuel with a higher cetane number, which allows the fuel in a compression ignition engine to cause a shorter auto-ignition delay time and therefore an earlier start of the combustion process. This phenomenon also results from the lack of changes made to the engine controller during the tests. For HVO, the calorific value is clearly higher than for DF, which may significantly affect engine parameters such as power and torque, as well as the pressure in the combustion chamber. To conclude, for the tested Perkins engine powered by HVO fuel, no significant differences were observed in engine performance or in the combustion process. Based on the results, the authors claim that HVO can replace diesel fuel even without major modifications or changes in engine settings. It should be noted that HVO fuel should also meet the other requirements described in the applicable standard for the physical and chemical properties of diesel fuel or the World Fuel Charter.