## 3 Engine Investigations  
### Test Facilities and Procedures  
The effect of back pressure on engine performance parameters was carried out experimentally on a 1035-L6-DT series commercial vehicle diesel engine manufactured by the Roman truck company; the main characteristics of this engine are given in Table 1.  
The engine performance was measured in the Engine Testing Laboratory of the Road Vehicle Institute, Brasov on a DC 300 kW/3000 rpm dynamometric test-bench produced by MEZ-Vsetin. The engine layout and the positions of the relevant sensors are illustrated in Figure 1. The bench was instrumented with temperature sensors for exhaust gas, air and cooling fluid, pressure sensors for charge air, ambient air, oil pressure and EBP, flowmeters for air and fuel, a tachometer for engine rotational speed, and an exhaust gas op \begin{table}
\begin{tabular}{l c} \hline \hline
**Characteristic** & **Value** \\ \hline Type & Diesel, 4-stroke, turbocharged, direct injection \\ Cylinder configuration & 6-cylinder, in line \\ Bore \(\times\) stroke [mm] & \(121\times 150\) \\ Compression ratio & 16:1 \\ Rated power [kW] & 172 \\ Rated speed [rpm] & 2200 \\ Maximum torque [N-m] & 900 \\ Maximum torque speed [rpm] & 1600 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Engine technical data. system, and the speed was adjusted by the dynamometer. The EBP was externally applied by a mechanical throttle valve installed on the exhaust system of the test-bench, 2 m downstream the turbine. The adjustment of the EBP was performed manually with intermediate positions calibrated after the maximum EBP values were fixed at rated speed and full load. The exhaust gas temperature was measured using a K-type thermocouple (chromel-alumel) inserted in the exhaust gas duct by means of a bayonet coupling. The air flow rate was measured by a laminar flow MERIAM flowmeter, the fuel flow rate was measured with a fuel balance produced by AVL List, using the chrono-gravimetric method by measuring the time required to consume 0.5 kg of fuel in any steady point. A digital inductive tachometer was used to read engine speed. The exhaust gas and charged air pressures were read on a Hg U-type manometer and piezo-resistive transducers. In addition to the measurement of engine performance characteristics, the smoke opacity was measured with a Hartridge Dismoke 435 opacimeter produced by AVL List that has an effective length of measurement tube of 430 mm, the readings are expressed both in HSU (Hartridge Smoke Unit) and light absorption coefficients (m\({}^{-1}\)). All the instruments met the accuracies imposed by engine testing standard given in Appendix A, Table A1, along with calculated measure uncertainties. For this engine type, the load was correlated with injection fuel flow, which was adjusted by changing the position of the fuel pump rack. No after-treatment device was mounted on the exhaust duct, as the effect of such was simulated by externally-induced back pressure. Each test started after the engine was warmed up and included a full load and part load tests at rated engine speed with the fuel pump rack fully opened. There were an additional six tests aiming to find part-load curves at intermediate positions of the fuel pump rack. After warming-up, at each set point, the engine was run for 6 min to reach the steady state condition. The experiments were triplicated and averaged to get more accurate values. A test matrix of 42 distinct operation modes (7 speeds and 6 loads) were chosen to provide a wide range of engine performance map. The speed was adjusted from 1000 to 2200 rpm in 200 rpm increments; the load was set to 100% (full load) and part loads of 90%, 70%, 58%, 48%, and 30%. In their engine specifications, vehicle manufacturers declare an allowable EBP at which their engine operates and which should not be exceeded. For the tested engine, this limit is 500 mm water column (WC) or 4.9 kPa; as a result, during the test, the minimum value of the externally applied EBP was set to 500 mmWC, and this was considered the reference.