**A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines**

Antonio Mariani, Biagio Morrone and Andrea Unich

*Dept. of Aerospace and Mechanical Engineering - Seconda Universitá degli Studi di Napoli Italy* 

#### **1. Introduction**

16 Fossil Fuel and the Environment

This work was a cooperative research project with the Japan Petroleum Energy Center. We thank Prof. M. Arai and Prof. K. Amagai of Gunma University for helpful suggestions.

Hayashida, K., Amagai, K., Satoh, K. & Arai, M. (2006). Experimental Analysis of Soot

Hepp, H., Siegmann, K. & Sattler, K. (1995). New Aspects of Growth Mechanisms for

Iwama, K. (2005). The Diversification of Energy Source in the New Era. *Journal of the Japanese Association for Petroleum Technology*, Vol.70, No.2, pp.125-131, ISSN 0370-9868 Kidoguchi, Y., Yang, C. & Miwa, K. (2000). Effects of Fuel Properties on Combustion and

Kök, M.V. & Pamir, M.R. (1995). Pyrolysis and Combustion Studies of Fossil Fuels by

Shaddix, C.R. & Smyth, K.C. (1996). Laser-Induced Incandescence Measurements of Soot

Zerda, T.W., Yuan, X., Moore, S.M. & Leon y Leon, C.A. (1999). Surface Area, Pore Size

Zhao, H. & Ladommatos, N. (1998). Optical Diagnostics for Soot and Temperature

Flames. *Combustion and Flame*, Vol.107, No.4, pp.418-452, ISSN 0010-2180 Zanier, A. (1998). Thermal-Oxidation Stability of Motor Gasolines by Pressure d.s.c.. *Fuel*,

Formation in Sooting Diffusion Flame by Laser-Induced Emissions. *Journal of Engineering for Gas Turbines and Power, Transactions of the ASME*, Vol.128, No.2,

Polycyclic Aromatic Hydrocarbons in Diffusion Flames. *Chemical Physics Letters*,

Emission Characteristics of a Direct-Injection Diesel Engine. *SAE paper 2000-01-1851*

Thermal Analysis Methods. *Journal of Analytical and Applied Pyrolysis*, Vol.35, No.2,

Production in Steady and Flickering Methane, Propane, and Ethylene Diffusion

Distribution and Microstructure of Combustion Engine Deposits. *Carbon*, Vol.37,

Measurement in Diesel Engines. *Progress in Energy and Combustion Science*, Vol.24,

**6. Acknowledgement** 

pp.241-246, ISSN 0742-4795

pp.145-156, ISSN 0165-2370

Vol.233, No.1-2, pp.16-22, ISSN 0009-2614

Vol.77, No.8, pp.865-870, ISSN 0016-2361

No.12, pp.1999-2009, ISSN 0008-6223

No.3, pp.221-255, ISSN 0360-1285

**7. References** 

In the last ten years, the number of natural gas (NG) vehicles worldwide has rapidly grown with the biggest contribution coming from the Asia-Pacific and Latin America regions (IANGV, 2011). As natural gas is the cleanest fossil fuel, the exhaust emissions from natural gas spark ignition vehicles are lower than those of gasoline-powered vehicles. Moreover, natural gas is less affected by price fluctuations and its reserves are more evenly widespread over the globe than oil. In order to increase the efficiency of natural gas engines and to stimulate hydrogen technology and market, hydrogen can be added to natural gas, obtaining Hydrogen - Natural Gas blends, usually named as HCNG.

This chapter gives an overview of the use of HCNG fuels in internal combustion engines. The chemical and physical properties of hydrogen and natural gas relevant for use in internal combustion engines are described. Then a survey on the impact of hydrogen on natural gas engine performance and emissions is presented with reference to research activities performed on this field.

#### **2. Data reduction**

In this section the main physical quantities used in this chapter are presented and discussed.

The stoichiometric air-fuel ratio on mass basis (AFR*stoich*), defined in equation 1, is the mass of air needed to fully oxidize 1 kg of fuel, while AFR is the ratio between air and fuel mass flow rates, equation 2. The ratio between the actual AFR and the AFR*stoich*, is the relative air-fuel ratio, equation 3. If *λ >* 1 the mixture is *lean* and the oxidation takes place with excess of air respect to the stoichiometric amount; for *λ* values lower than 1 the mixture is *rich*, and the fuel oxidation is not complete. The ratio 1/*λ* is defined as the equivalence ratio *φ*, equation 4.

$$AFR\_{stoich} = \left(\frac{m\_d}{m\_f}\right)\_{stoich} \tag{1}$$

$$AFR = \frac{m\_a}{m\_f} \tag{2}$$

$$
\lambda = \frac{AFR}{AFR\_{stoich}}\tag{3}
$$

**3. Natural gas**

The main natural gas constituent is methane and the composition is strictly dependent on the origin gas field. Table 1 shows the composition of a natural gas sample obtained by the Italian

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 19

Natural gas has been widely investigated as fuel for road vehicles because of its lower impact

**Constituent Composition [% vol.]**

Ristovski et al. (2004) performed an experimental activity on a passenger car converted to operate either on gasoline or on compressed natural gas (CNG). Fuelling the engine by CNG, both regulated (CO, NOx and HC) and unregulated emissions (PAHs and formaldehyde) were

Prati, Mariani, Torbati, Unich, Costagliola & Morrone (2011) tested a bifuel passenger car fuelled alternatively by gasoline and natural gas on a chassis dynamometer over different driving cycles, in order to evaluate the effects of fuel properties on combustion, exhaust emissions and engine efficiency. The results showed that gasoline produced CO emissions higher than NG over the real world Artemis driving cycles, as a consequence of mixture enrichment during load transients. A detailed description of the driving cycles is reported in Barlow et al. (2009). Over the type approval New European Driving Cycle (NEDC), NG involved higher HC emissions compared to gasoline as a consequence of the higher light-off temperature for the catalytic oxidation of CH4, which is the major constituent of HC when the vehicle is fuelled by NG, while there were no differences over the Artemis driving cycles which were performed after a warming up conditioning of the vehicle. NOx emissions were higher for gasoline over all the test cycles. CO2 emissions for CNG showed a reduction between 21% and 29% over the tested driving cycles as a consequence of the reduced carbon content of the fuel and the lower fuel consumption on mass basis. A 5% fuel consumption reduction, expressed in MJ/km, is observed over the NEDC for the CNG respect to gasoline, while for the Artemis the reduction ranges between 10% and 22%. The higher gasoline consumption is the consequence of the mixture enrichment during transients. Particulate emissions referred to gasoline were higher than NG ones over the NEDC and comparable over the Artemis. Particle number observed was also higher for gasoline, with the exception

Methane 88.98 Ethane 6.85 Propane 1.27 Butane 0.24 Pentane 0.04 Hexane 0.003 Nitrogen 0.96 Carbon dioxide 1.61

distribution network, determined by means of gas chromatographic analysis.

on the environment than gasoline and more widespread resources.

Table 1. Example of natural gas composition.

lower than gasoline.

of the Artemis Motorway.

$$
\phi = \frac{1}{\lambda} \tag{4}
$$

Equation 5 defines the indicated mean effective pressure (imep), an engine parameter which evaluates the work obtained by an engine cycle, *p dV*, divided by the engine displacement. The Coefficient of Variation of imep, COV*imep*, is the ratio of the standard deviation of the indicated mean effective pressure and the average imep over a representative number of cycles, equation 6.

$$imp = \frac{1}{V\_d} \oint p\,dV\tag{5}$$

$$COV\_{imp} = \frac{\sigma\_{imp}}{\dot{image}\_{avg}}\tag{6}$$

In case the effect of mechanical efficiency has to be taken into account, the brake mean effective pressure (bmep) is considered. In 4-stroke engines, the bmep is calculated from the torque measured at the engine shaft, according to equation 7:

$$bmep = \frac{T \cdot 4\pi}{V\_d} \tag{7}$$

The stoichiometric reaction equation of a methane-hydrogen blend reads as:

$$\left(\left(\text{aCH}\_4 + \beta \, H\_2\right) + \left(2\,\text{a} + \frac{\beta}{2}\right)\left(\text{O}\_2 + 3.76 \, N\_2\right) \rightarrow \text{aCO}\_2 + \left(2\,\text{a} + \beta\right)H\_2O + \left(2\,\text{a} + \frac{\beta}{2}\right) 3.76 \, N\_2\right) \tag{8}$$

where *α* + *β* = 1. The quantities *α* and *β* represent the mole per each species in the blend, and it is immediate to observe that the reduction of the C/H ratio, compared to pure methane, brings about a theoretical reduction of the CO2.

The burning velocity represents a main property for the combustion characteristics of the fuels and is defined as the velocity at which unburned gases move through the combustion wave in the direction normal to the wave surface (Glassman & Yetter, 2008). The laminar burning velocities can be obtained using the following equation 9 (Mandilas et al., 2007) being S*<sup>s</sup>* the unstretched flame speed, *ρ<sup>b</sup>* and *ρ<sup>u</sup>* the burned and unburned gas densities. Equation 10 relates the unstretched flame speed, the stretched flame speed S*n*, the stretch rate *κ* and the Markstein length L*b*.

$$
\mu\_l = \mathcal{S}\_s \frac{\rho\_b}{\rho\_u} \tag{9}
$$

$$
\mathcal{S}\_s - \mathcal{S}\_\hbar = \kappa L\_\hbar \tag{10}
$$

The stretch rate *κ* is calculated from the position of the flame front, *R* = *R*(*t*), with the following equation 11 (Chen, 2009):

$$\kappa = \frac{1}{R} \frac{d\mathcal{R}}{dt} \tag{11}$$

The Markstein length characterizes the variation in the local flame speed due to the influence of external stretching and determines the flame instability with respect to preferential diffusion (Markstein, 1964).
