Keywords

1 Introduction

An efficient method to increase thermal efficiency and specific power of the spark ignition engines is supercharging [1, 2]. Supercharging is common for diesel engines, but for SI engines becomes restrictive because of the main disadvantages represented by abnormal combustion phenomena with knock, exhaust gases temperature increasing, engine thermal and mechanical stresses increasing [3]. Supercharging is a significant method for the specific power increasing of the spark ignition engine which can be assured by the increase of indicated mean effective pressure. This efficient method allows the use of the downsizing and down speeding concepts which applied at the SI engines represent modern solutions for a lower displacement engines development (compact gauge and low costs), with a lower speed for maximum power/maximum torque comparative to aspirated engines (for the same or higher power/torque values), with favourable influences on engine thermal and mechanical stress, thermal efficiency, pollutant emissions and wear. Supercharging was considered a common method used for IMEP increase for diesel engine only. The main issues of spark ignition engine supercharging are represented by possibility of knocking phenomena appears, exhaust gases temperature increases, engine thermal–mechanical stress increases.

Nowadays modern management of SI engine running allows the supercharging use also for the spark ignition engines. Thus, the engine operation electronic control can assure every time an optimal correlation between supercharging pressure—compression ratio—spark ignition timing—supercharging air temperature—dosage-exhaust gases temperature that can allows the spark ignition engine operation without knocking combustion and with remarkable energetically and polluting performance. For example, at the supercharging use the engine thermal efficiency could be increased by 20 %, the specific power could be increased by up to 20 %, and the pollutants emission level could be reduced (is estimated to obtain a reduction of over 5 % for NOX emissions, a 15 % reduction of CO emission and a considerable reduction in CO2 emission) [4].

During the last time the supercharging of SI engines became the most efficient method of increasing their performance from the point of view of energetically and polluting terms [5, 6].

Due to sever national and international pollutant regulations for automotive SI engine, especially for CO2, the research programs are leaded for alternative fuels use and for engine thermal efficiency improvement which is directly related to CO2 emission. Among the alternative fuels used for automotive SI engine bioethanol represents a viable fuel due to its unlimited manufacturing sources. The use of bioethanol at supercharged SI engines assures an efficient intake air cooling effect due to its higher heat of vaporization. The intake air cooling effect leads to a volumetric efficiency improvement and reduces the knock risk developed. The higher bioethanol octane number (RON 106-114) increases the auto-igniting resistance of the end-gas and it may be considered as an efficient antiknock agent. The good burning properties of the bioethanol comparative with the gasoline (greater burning speed, smaller carbon content and larger oxygen content at molecular level) assure the combustion efficiency increases. These advantages of the bioethanol use are added at the supercharged SI engine typically distinguished advantages: thermal efficiency and specific power increase and pollutant emissions decrease.

In order to use bioethanol as alternative fuel for a supercharge SI engine the following methods can be use [7, 8]:

  1. 1.

    The engine can be fuelled with bioethanol- gasoline blends by:

    1. (a)

      intake manifold bioethanol-gasoline blends injection (MP injection)

    2. (b)

      in-cylinder direct bioethanol-gasoline blends injection

  2. 2.

    The injection of bioethanol in the intake manifold after compressor (an intense local cooling effect of the compressed air is achieved) and a separately injection of gasoline in the intake manifold in intake valve port or direct injection in cylinder.

The paper objectives are the increase of the specific power, of the thermal efficiency and the decrease of the pollutants emissions level for an automotive spark ignition engine by supercharging method and bioethanol fuelling. The authors have the goal to apply supercharging method to a serial automotive aspirated spark ignition engine. The SI engine supercharged was fuelled with ethanol- gasoline blend (E20) by MP injection.

1.1 Engine In-Cylinder Thermo-Gas Dynamic Processes Simulation

Engine in-cylinder processes simulation was developed for a 1.5 l engine. In order to define the energetically performance and the cycle performance parameters, a zero dimensional and unizonal physic-mathematical model was developed [9]. There have been considered the following hypothesis:

  1. (a)

    The motor fluid is treated as a perfect gas.

  2. (b)

    The system is thermodynamically homogenous, thus its every point has the same temperature and pressure.

  3. (c)

    The entire combustion developers neglecting the chemical reactions that leads to intermediate reaction products.

The model takes into consideration the local cooling effect produced by ethanol vaporisation and relieves the influence of this parameter on cylinder filling and engine performances. The model uses a Vibe combustion formal law and takes into consideration the heat transferred to the walls, which has been calculated with Woschni formula (1994).

For knock avoiding, the combustion duration was established shorter than end-gas auto ignition delay, parameters being evaluated by Douaud and Evzat equation [2]. For program calibration the experimental investigations results were used and the considered model hypotheses were verified. The modelling processes was developed for the aspirated engine and for the supercharged engine designed and built in the laboratories of the Department of Thermotechnics, engines, thermal equipments and refrigeration installations from University Politehnica of Bucharest at different engine operating regimens. Different supercharging pressures, ps—absolute pressure—and temperatures, Ts, of the blower exhaust cooled air were taken into consideration.

For knock avoiding the reach dosages in the area λ = 0.8–1 and cooling of the inlet air were used. For each operation regime, the spark ignition timing was set up for knock avoiding. The modelling results for full load and speed of 2,500 rpm are shown in Figs. 2, 3, 4.

2 Experimental Investigations

The experimental researches were carried on an automotive SI engine 1.5 Ll. The engine mounted on a test bench (Fig. 1) was equipped with the next necessary instruments for measuring operations: AVL ALPHA 160 eddy current dynamometer equipped with throttle actuator that work in parallel with the dyno in order to operates the throttle, real time AVL data acquisition system for processing and storage of measured data’s, AVL in-cylinder pressure transducer line, AVL gas analyzer, Khrone Optimass mass flow meter, engine inlet air flow meter, thermo resistances for engine cooling liquid temperature, engine oil and air intake temperatures and thermocouples for exhaust gas temperature, manometer for air pressure from engine intake manifold. All instrumentation was calibrated prior to engine testing. The experimental investigations were carried out on the aspirated engine and also the supercharged engine. In this paper the experimental investigation results for a turbocharged E20 fuelled engine are shown. (The supercharge system is equipped in this first stage with a variable-geometry turbine [10]. As novelty character of the research, in the next research stage the engine will be equipped with a compressor mechanically actuated with variable transmission ratio for supercharge pressure modification. The bioethanol will be injected into the compressed air in order to assure an efficient cooling).

Fig. 1
figure 1

Test bed schema. AGE exhaust gas analyzer; AS charge amplifier; B battery; ca intake manifold; CAD data acquisition computer; ce intake manifold; CF dyno power cell; CG fuelling system computer; CIG injectors actuation; ct three way catalyst; DA air flowmeter; DC fuel flowmeter; EGR exhaust gas recirculation valve; F eddy current dyno; FC fuel filter; IND Indimodul 621 data acquisition unit; IT temperature indicators; M Daewoo 1.5 spark ignition engine; MP supercharging pressure manometer; ORS throttle; PCF dyno command panel; R engine cooler; RI intercooler; RC fuel reservoir; SA power supply; SCP throttle actuator servomotor; SRAF dyno cooling system; st gas analyzer speed sensor; TC turbo compressor; tf dyno cooling water temperature sensor; TF dyno speed transducer; tp cylinder pressure transducer; TPU angle encoder; UCP principal electronic control unit; UCS secondary electronic control unit; UPF dyno power unit; UPS throttle actuator servomotor power unit; VE cooling electric fan for intercooler; x electronic emitter–receptor

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Experimental research’s carried out to obtain fuel consumption characteristics for different speeds and engine full load and to determinate the energetically and pollutant engine performance. At each engine operating regime for maximum pressure limitation and knock avoiding, the dosage, spark ignition timing and supercharge pressure were modified. Thus, an optimal correlation between supercharge pressure-compression ratio- dosage-spark timing-exhaust gases temperature was establish. Based on fuel consumption characteristics were obtained some graphic representations for different parameters such as: effective power, indicated specific fuel consumption (ISFC), maximum pressure, pollutants emissions level (HC, CO, and NOx) versus air-fuel ratio λ, at different boost pressure values and engine operation regimes.

3 Theoretical and Experimental Investigations Results

The results of theoretical and experimental investigations presented in Figs. 2, 3, 4 show good correlation between them.

Fig. 2
figure 2

Engine power versus air–fuel ratio at full load and 2,500 rpm—1.4 bar boost pressure

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Fig. 3
figure 3

ISFC versus air–fuel ratio at full load and 2,500 rpm—1.4 bar boost pressure

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Fig. 4
figure 4

Maximum pressure versus air–fuel ratio at full and 2,500 rpm—1.4 bar boost pressure

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Supercharging pressures used are in the range of 1.3–1.8 bar. Comparative to the aspirated engine, the maximum pressure increases with almost 100 % for a supercharging pressure (ps) of 1.8 bar at engine full load and 4,800 rpm (speed of maximum power). High value of maximum pressure leads to the limitation of supercharging pressure at 1.4 bar when the increases of maximum pressure value (~with 50 % comparative to classic solution) is acceptable for the engine reliability. For this supercharging pressure, the engine maximum power increases with 33 % (from 65 kW for aspirated engine to 86 kW for supercharged engine). In order to avoid the knocking and to limit the maximum pressure, the ignition angle value optimization was achieved. The results of the theoretical and experimental investigations are presented in the Figs. 2, 3, 4, 5, 6, 7. The maximum torque value of the aspirated engine obtained at 3,000 rpm is establish at 2,500 rpm for the supercharge engine and the maximum power at 3,700 rpm comparative to 4,800 rpm for aspirated engine. Thus, by supercharging use the down speeding concept can be applied and the mechanical stresses and engine wear are reduced.

Fig. 5
figure 5

CO emission versus air–fuel ratio at full load full load and 2,500 rpm—1.4 bar boost pressure

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Fig. 6
figure 6

HC emission versus air–fuel ratio at full load 2,500 rpm—1.4 bar boost pressure

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Fig. 7
figure 7

NOx emission versus air–fuel ratio at full load and 2,500 rpm—1.4 bar boost pressure

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The theoretical and experimental results at full load regime and 2,500 rpm for the aspirated engine and also for the supercharged one (1.4 bar boost pressure) are presented in Figs. 2–7.

Thus, a good correlation between the theoretical and measure results is observed from the Figs. 2, 3 (continuous line-measured results; discontinuous line-calculated results).

By supercharging the engine power increases with ~55 % for all air-fuel ratios, Fig. 2, engine efficiency being improved, Fig. 3. For stoichiometric dosage, the ISFC decreases with ~3 % by supercharging for gasoline fuelling and with ~10 % for E20 fuelling. For knock avoiding at supercharged engine much reach dosages were used (λ = 0.8–1). At the same dosage, for bioethanol E20 use the power of the supercharged engine increases with ~10 % comparative to the supercharged gasoline fuelled engine due to the cooling effect produced by bioethanol vaporisation and to the combustion improvement.

Bioethanol fuelling allows engine operation at much leaner mixtures without knocking running, the power for stoichiometric dosage of the supercharge engine being obtained for a much leaner dosage, λ = 1.1, but for a 10 % lower specific fuel consumption. The supercharging method also leads to the increase of the maximum gas pressure with unfavourable influences on mechanical stresses. For maximum gas pressure limitations, the authors limits the supercharging pressure at 1.4 bar (the theoretical and experimental investigations were developed for a wide area of supercharging pressures, 1.1–1.8 bar) for which, at stoichiometric dosage and also for reach dosages, the maximum pressure rises with 18 % for gasoline fuelling and with 23 % for E20 fuelling. In the area of lean dosages, λ = 1.2, for E20 fuelling, the supercharged engine in-cylinder maximum pressure is at the same level comparative to gasoline engine. Authors appreciate that at these levels of maximum pressure values, the engine thermo-mechanical stresses are not critical and engine design major modifications are not required. The improvement of the combustion process by supercharging use at spark ignition engine leads to the favourable effect of CO and HC emissions level decreases for gasoline fuelled engine (with 10 and 13 %, respectively, at stoichiometric dosage and more significantly for reach dosages). For E20 the reduction of those two emissions is more accentuated, due to a lower C content and better combustion properties for bioethanol.

Regarding the NOx emission a reduction of 20 % is assured at stoichiometric dosage and reaches dosages, at gasoline and also much more at E20 fuelling, because of the local cooling effect produced by bioethanol vaporisation. Instead, in the area of much leaner dosages, the NOx concentration pronounced increases, over the level established for aspirated gasoline engine, because of the oxygen content increases. In terms of theoretically investigations, authors appreciates that thru the extension of the lean dosages area, λ = 1.2, the NOx emission concentration significantly decreases, with the effect of supercharged engine power reduction (power that always remains bigger comparative to the aspirated engine).

4 Conclusions

From the theoretical and experimental investigations results analyze the following conclusions can be formulated:

  1. 1.

    SI engine supercharging is a method to obtain engine efficiency and specific power/torque performance increasing.

  2. 2.

    At bioethanol use a supplementary increase of the engine efficiency and power/torque is obtained.

  3. 3.

    The utilisation of the supercharge method for SI engine allows the significant reduce of the CO and HC emissions.

  4. 4.

    At bioethanol use for supercharge engine the reduction of CO and HC emissions are more significant due to much better combustion properties of bioethanol that allows lean dosages use.

  5. 5.

    SI engine supercharging method use leads to a significant decrease of NOx for gasoline supercharged engine, in the area of stoichiometric and reach dosages.

  6. 6.

    For bioethanol use at supercharged engine the reduction of NOx is more important in the area of λ > 1.1 dosages, were the strategy of engine load qualitative adjustment can be applied.

  7. 7.

    The thermo-mechanical stresses of the supercharged engine are much higher comparative to the aspirated engine and can be controlled by supercharging pressure limitation, without engine design major modification.

  8. 8.

    Knock is the most important limiting factor of the supercharging engine. An optimum correlation establish between air boost pressure-air boost temperature—compression ratio-dosage-spark ignition advance-exhaust gas temperature-brake mean effective pressure, brake specific fuel consumption leads to the avoiding of knocking phenomena.

  9. 9.

    Supercharging represents an efficient method of he engine downsizing and down speeding concepts use.

  10. 10.

    Bioethanol can be defined as an efficient agent for knock avoiding.