# Combustion and Emission Characteristics of a Biodiesel-Hydrogen Dual-Fuel Engine
## 3 Results  
The impact of hydrogen addition for engine parameters was determined based on the pressure curve analysis. For each operating point, the pressure curve for 200 engine cycles was recorded, and the combustion process was assessed on the basis of the average course from the set. The analysis of the influence of hydrogen on the course of combustion pressure and pressure rise rate, as well as on heat release rate was done. The normalized heat release rate was then used to determine the characteristic combustion phases. Figure 2 presents pressure curve for the analyzed hydrogen energetic shares. Along with the increase in the energetic share of hydrogen, an increase in the maximum pressure was observed, which was achieved closer to the top dead center (TDC). For the hydrogen energetic share of 38%, the pressure increased by 1.2 MPa compared to the pressure of the diesel engine. The p\({}_{\text{max}}\) value was also 4 deg of crank angle (CA) earlier than a conventional engine. For the high value of energetic share for hydrogen, the start of combustion took place very early and the combustion process, as can be seen on pressure increase rate curve, occurred in two stages. It should also be noted that for all H\({}_{2}\) shares, the peak values dp/d\(\varphi\) were within the allowable range for piston engines, i.e., below 1 MPa/deg. Figure 3 presents heat release rate for an engine combusting biodiesel and hydrogen. The analysis of heat release in the engine is a reliable source of information about the combustion process. From the heat release rate (HRR) curves, it was found that with the increase of H\({}_{2}\) energy share, the diffusion combustion phase decreases and kinetic combustion phase increase at the same time. It can even be stated that with an increase in the proportion of hydrogen in the place of the diffusion phase, an intermediate combustion phase appears between the kinetic and diffusion phases, which is shown by an additional local maximum on the HRR curve. For a 38% hydrogen share, the second peak on the HRR curve is even higher than the first peak of kinetic combustion which is typical for an engine. With a high proportion of H\({}_{2}\), biodiesel combustion has two visible stages. On the basis of the normalized heat release rate curves, the combustion stages were determined, i.e., the ignition delay time and the duration of combustion. In the field of IC engines, these times are defined in CA degrees, which allows comparison of results for different engines. Both physical and chemical processes affect the ignition delay time. The first processes depend primarily on the  
Figure 3: Heat release rate and normalized heat release for different energetic shares of hydrogen. Figure 2: Combustion pressure curve and pressure rise rate for different energetic shares of hydrogen. quality of fuel atomization, the method of its delivery to the combustion chamber and gas dynamics processes in the engine cylinder. These factors cause the so-called physical ignition delay. The second processes depend on the chemical properties and depend on the quality of the fuel, C/H ratio or the share of oxygen in the molecular structure, its heat of evaporation value, ignition temperature, laminar flame speed (LFS) or lower heating value (LHV). Both phenomena, i.e., physical and chemical delay, occur simultaneously. In practice, most often the ignition delay in a piston engine is assumed to be the period, expressed in degrees of CA, from the beginning of diesel injection until the release of 10% of heat. The next stage is the duration of combustion counted from the release of 10% of heat to the point of release 90% of heat. In the combustible mixture hydrogen is premixed, it is ignited and combusted earlier than biodiesel, which can be seen from Figure 3 (the two-peaks). The combustion product of hydrogen is steam. The steam mixed with air and result in inhomogeneous to the biodiesel diffusion flame. This, among other things, affects the shape of pressure courses and its derivatives.