# Medium-Energy Synthesis Gases from Waste as an Energy Source for an Internal Combustion Engine
## 3 Experimental Methods
### Analysis of the Pressure in the Cylinder  
The measurement of pressure profiles in the internal combustion engine was performed at the operating speed of the micro-cogeneration unit of 1500 L/min. The pressure was measured using a sensor system from Kistler, Switzerland. The cylinder pressure was measured using a piezoelectric pressure transducer integrated in a spark plug from Kistler (6118 CC-4 CQ02-4-1). Corrections of the dynamic pressure profile in the cylinder were carried out by detecting the pressure profile in the suction line, in the region of the bottom dead centre at the time of opening of the suction valve, when equivalence of the pressure values was assumed. The absolute value of the intake manifold pressure was read using a piezoresistive pressure transducer from Kistler (4075A10). The current position of the crankshaft was measured using a Kistler 2613B1 encoder. For fuel burn-up analysis, the actual spark ignition moment was also recorded for each cycle using a separately developed diode-optocoupler sensor, which was connected in parallel with two BOSCH P65-T combined ignition coils with a maximum spark energy of 65 mJ, [46]. An individual program for processing and evaluating measured data was developed using the general programming language Matlab. The analysis of the course of fuel burning out process was based on a single-zone zero-dimensional thermodynamic model [47; 48]. The analysis of the course of fuel combustion and heat release is based on the Rassweiler-Withrow method. This method is based on the knowledge that the pressure increase in the combustion chamber of the engine consists of a partial component of the pressure arising from the combustion itself and a partial component arising from the movement of the piston in the cylinder. In general, the method is based on the first law of thermodynamics in the following form:  
\[dU=dQ-dW+\sum_{i}h_{i}dm_{i}, \tag{1}\]  
where: \(dU\)--change of internal energy of matter in the system; \(dQ\)--heat delivered to the system; \(dW\)--the work produced by the system; \(h_{i}dm_{i}\)--\(i\)-th component of enthalpy of mass flow across system boundaries. By detailed writing and modifying the above Equation (1), we get the following form:  
\[dQ_{ch}=\frac{1}{\kappa-1}\;Vdp+\frac{\kappa}{\kappa-1}pdV+\left(u-\frac{rT}{ \kappa-1}\right)dm_{clk}-\sum_{i}h_{i}dm_{i}+dQ_{ht}. \tag{2}\]  
During combustion, the last three terms of Equation (2) are assumed to be zero, and subsequently the final pressure increment shape is obtained from two partial increments:  
\[dp=\frac{\kappa-1}{V}dQ-\frac{\kappa p}{V}dV=dp_{c}+dp_{p}. \tag{3}\]  
In Equation (3), the first term (\(dp_{c}\)) represents the pressure change owing to combustion and the second term (\(dp_{p}\)) represents the incremental change owing to the change in volume. The start and end of combustion (SOC, EOC, respectively) were determined by the change in entropy during combustion. This method is explained in more detail in the literature [49]. The second method determining the beginning and end of combustion was implemented on the basis of the deviation of the combustion curve from the compression or expansion curve in the logarithmic p-V diagram. From each measured pressure profile (approximately 195 consecutive cycles), statistical analysis was also evaluated to determine the coefficients of variation (\(COV\)) of the various cyclically repeating parameters. It is calculated as the ratio of the standard deviation to the arithmetic mean of the investigated parameter, using the following formula:  
\[COV=\frac{\sqrt{\frac{1}{n-1}\sum_{i=1}^{n}x_{i}-\overline{x}}}{\overline{x}}.100 \;[\%]. \tag{4}\]  
Cycle variability deals with the uniformity of engine operation as well as the overall life of the internal combustion engine and also affects the engine performance parameters. All synthesis gases were tested in the speed range from 1200 to 2200 1/min, with stoichiometric mixture and at full load. The spark advance angle was optimized for each engine operating mode to achieve maximum torque.