2.0 LITERATURE REVIEW
This chapter of the report will go through the detailed literature review described in the introduction. The review starts by giving details about the Diesel engines by recognizing their importance in all types of industries now a days. Followed by detailed review about the effect of ambient conditions (in QATAR) including humidity and air temperature on engine performance, as well as their relationship to engine volumetric efficiency. Moreover, detailed discussion on the working mechanism of A/C in vehicles and how such systems may affect engine performance.
Finally the selected experimental method for this project will be discussed in depth in light recent experimental tests in enhancing induced ambient conditions on ICE.
2.1 Diesel engine
Diesel engines were traditionally been underestimated in terms of power and class. They were limited to be utilized in trucks, ambulances, taxis and vans. After improving the injection system and became more sophisticated, there were almost 65,000 diesel cars sold in 1985 in UK compared with only 5,380 in 1980 .
The main advantages of Diesel engines over gasoline engines is their lower running cost which is due to the fact that diesel engines acquire greater efficiency due to their high compression ratios (12-24). Diesel prices stand a crucial factor in running diesel engines; hence, diesel prices in Qatar to be specific, is relatively lower than the gasoline prices. As a result, a chance in utilizing diesel-powered vehicles.
Below is the sequence of how diesel engine works;
2.2 Engine performance parameters
Engine performance can be characterized through several parameters. In order to develop internal combustion engine, all the affecting parameters shall be taken into consideration to fulfill governmental regulations in terms of emissions and to have an overall comparable engine efficiency.
2.2.1 Engine power
There are two common terminologies that defines engine power; maximum rated power and normal rated power. Maximum rated power refer to the highest power that an engine is allowed to achieve for short periods of time. Moreover, normal rated power refer to the highest power developed in continuous operation. The net power output depends on the size and design of the engine keeping in mind the running speed and the load it encounters. It can be expressed in kilowatts or horsepower. There is another important terminologies that best describe engine power; indicated and brake power.
Indicated power is defined as the power achieved by combustion of fuel inside the cylinder without having in mind the power loss in the mechanical components due to friction. Thus brake power is the actual power delivered to the driving wheels including all losses. Therefore, mechanical efficiency is considered to be the ratio of brake power to the indicated power as per equation 1:
Mechanical Efficiency= Pb/Pi .. Equation (1)
Where, Pb: brake power and Pi: indicated power
2.2.2 Brake Specific Fuel Consumption
BSFC, is the rate of the fuel consumed to the brake power produced; hence, it is another way to measure the efficiency of the engine. It has units of grams of fuel per kilowatt-hour (g/kWh). Typically BSFC is poor at low load; moreover, it is directly affected by the ambient temperature as per the experimental findings by Saber which was carried out in 2013 .
2.2.3 Break Mean Effective Pressure
Another useful engine performance measure is called mean effective pressure (mep). Such measure is purely theoretical and does not represent the actual cylinder pressure. It addresses some comparison measure between engines at rated parameters and others. It can be obtained by dividing the work per cycle by the displaced volume of the cylinder.
mep= (P*nr)/(Vd N) ..Equation (2)
nR: It is the number of crank revolutions for each power stroke per cylinder
Maximum break mean effective pressure of typical engine designs is well established and tabulated; therefore, actual bmep of any engine can be compared with this norm. As an instance, naturally aspirated four-stroke diesel engines, maxim bmep is in the range of 700 to 900 kPa. 
2.3 Effect of Ambient conditions on ICEs (QATAR CLIMATE)
Gulf climate and Qatar to be specific is well known with its hot and humid weather during most of the months. The most critical ambient conditions that highly affect engine performance are; ambient air temperature and relative humidity. Hence, inlet temperature has major effect on volumetric efficiency of the engine as will be discussed over the following sections.
Humidity is the amount of water vapor exist in air. As humidity increases, the engine will have more probability to misfire; hence, delay in combustion. On the other hand, lower humidity levels can affect the knocking margin as well as NOx emissions; however, lower engine efficiency.
Figure 2 describe the effect of humidity on engine efficiency; hence, the higher the humidity, the lower the efficiency and NOx level.
2.3.2 Inlet Air temperature
The temperature that is measured outside the engine compartment coming to the cylinder is called ambient inlet air temperature; hence, does not have direct effect on the engine efficiency, it affects the volumetric efficiency of the engine. Furthermore, starting from April up to November, warmest temperatures are illustrated, refer to below figure 3. 
Figure 3: Annual average temperature distribution in Qatar
With reference to figure 5, the higher the temperature, meaning the lighter the air being sucked into the system. This will result in lower combustion efficiency.  Therefor, it is recommended that relatively low inlet temperature should be used to gain more volumetric efficiency; hence, higher combustion efficiency.
Figure 4: Combustion efficiency is plotted against different temperatures and humidities
2.3.3 Volumetric efficiency Improvements
Volumetric efficiency is a ratio of the mass of air and fuel that comprises the cylinder medium divided by the by the mass that would occupy the displaced volume. The denser the air sucked to the engine, meaning more air to fuel ratio that will be utilized during combustion; hence, more power. There are many way to enhance the volumetric efficiency such as; using turbos or superchargers. As a result of forced induction volumetric efficiency can exceed 100%. Other way is by cooling the inlet ambient temperature using mini separate HVAC system or any AC modified systems which will be illustrated and discussed in details in the coming sections.
2.4 Air conditioning system in cars
One of the vital scopes of this research is to illustrate the effect of inlet ambient conditions on engine performance. Therefore, understanding such system in our cars shall give more clear vision of how it might be utilized for altering the condition of induced air. As a result, having denser air and eventually more power.
2.4.1 How does it work
Air conditioning system does not only cool the air, but also reduces the moisture content. All AC systems work similarly; whether they are utilized in buildings, fridge or oven civilian cars for cooling or heating purposes. It is a fact that in order to remove/add heat and moisture, energy is being consumed. Therefore, turning the AC in cars actually result in consuming diesel/ gasoline from the cars tank due to the extra work done on the cars engine. Hence, if the same AC unit is being modified and utilized to cool the inlet temperature coming to the engine compartment, this will result in more power which eventually outweigh the usage of normal AC inside the cabinet.
Controlling heat transfer efficiently between two regions of different temperatures requires special cyclic devices. Refrigerators (for cooling objectives), heat pump (for heating objectives) are the most well-known and used devices in modern times. Those devices operate by a cycle called vapor compression refrigeration cycle. Vapor compression refrigeration cycle consist of a compressor, condenser, throttling valve and evaporator
Ideally, cycle refrigerant (working fluid) is compressed at constant entropy at the compressor, leaving the compressor as superheated vapor. Then the superheated vapor enters a condenser in which heat is removed from the working fluid at constant pressure and temperature in order to condense the vapor into liquid. A fan is placed below the condenser in order to blow the heat to the external environment. After that the liquid refrigerant travels through a throttling valve which causes a significant drop in the pressure leading to vaporization of half of the liquid refrigerant leading to two phase refrigerant. Finally, the mixture travels through the evaporator that in turns heats up the mixture leading to evaporate the rest of the liquid at a constant pressure and then enters the compressor again to start the cycle again.
Yet the actual cycle deviates from the ideal cycle due to the increase of entropy during compression because of irreversibility or due to heat loss to the surrounding, pressure drop in pipes due to fluid friction and heat loss as well as pressure drop in condenser and evaporator as shown in Figures 6 and 7.
Figure 6: Ideal vapor compression refrigeration cycle
Figure 7: Actual vapor compression refrigeration cycle
In order to calculate the amount of heat gain at the evaporator, heat loss at the condenser and the amount of work input at the compressor the enthalpies at each stage must be determined. For ideal vapor compression refrigeration cycle as shown in Figure (6)
1. Work input at the compressor (no heat transfer):
2. Heat loss to the environment at the condenser (zero work):
3. At throttling process no work is done and no heat transfer, so h3=h4
4. Heat gain to the working fluid at the evaporator (zero work):
The above equations lead to the determination of the coefficient of performance (COP) which in turns an indication of the reliability of the heat pump or air conditioner.
COPC = (Cooling effect)/(Work input)=QL/Win=(h1-h4)/(h2-h1) . Equation (3)
2.4.2 Intercooling concepts and effects
Intercooler is a device used to cool the intake air coming to the engine compartment when using supercharged or turbocharged engine. It is a way to increase the volumetric efficiency as discussed earlier; hence, there are two ways of intercoolers; Air-to-Air and Air-to-Water.
As the air is compressed by either turbo/supercharger it gets hot very quickly. As the temperature gets very high, its density drops; therefore, by cooling the air it gets denser which allows higher air to fuel ratio coming towards the engine cylinder, thus giving more power .
126.96.36.199 Air-to-Air intercooler
It utilizes cool air coming from outside at very high speed to extract heat from the cooling fins of the intercooler which consequently reduces the temperature of the compressed air coming from the turbo/supercharger through its network of tubes,. Refer to figure 8
188.8.131.52 Air-to-water intercooler
This system uses water as the heat transfer agent that exchanges heat with the compressed air coming from the turbo/supercharger. Cool water is pumped into the intercooler and extracts heat from the compressed air. Afterwards, the heated water is then pumped back through a radiator via different circuit and the cooled compressed air is pushed into the cylinder. Refer to figure 9.
2.4.3 Experimental intercooling test rigs for ICEs
The intention of setting up experiments is to examine the concept of intercooling/specific ambient conditions with minim efforts in parts without imposing any big improvements to the concerned engine/vehicle. Many scientists and engineers conducted experiments that manipulated the ambient temperature/humidity. It was found that artificially controlling the ambient conditions to be energetically vital and effective in the sense of cooling the intake air whether the engine is naturally aspirated or equipped with supercharged/turbocharger. This paper will utilize a cooling method inline to pre-found results that will be briefly discussed in the coming paragraphs.
First experiment was conducted on a direct injection marine diesel engine of four cylinders arranged inline as described in below Table 1:
Table 1: Specifications of marine diesel engine
Charge air is forced into the intake manifold via a blower directed through an air conditioner at which air properties changes (i.e. temperature and relative humidity). The specifications of the A/C unit are listed in Table 2.
Table 2: External A/C system specifications
Set up matrix is summarized in figure 10 where data acquisition system along with engine cycle analysis software were utilized to ease acquiring data. The computerized laboratory is located in a coastal region to simulate real input data similar of a marine environment.
Figure 10: Schematic of experimental apparatus
It was concluded that engine performance and exhaust characteristics are highly effected by charge air temperatures rather than air humidity. Moreover, break specific fuel consumption, carbon monoxide and sulfur dioxide increases while brake torque and NOx decrease with the increase of both charge air temperature and humidity. 
Second experiment was targeting turbocharger SI engine for passenger cars as illustrated in Table 3 by modifying its existing air conditioning system; however, two experimental set ups were designed to study deeply the effect of the properties of charge intake air to engine performance. Set up one is illustrated in figure 11 which consists of two heat exchangers. The first one is a conventional water-to-air intercooler that cools the intake air to not more than 40oC then it undergoes further cooling by the extra intercooler ICE as shown on the same figure below. It utilized the existing parts of A/C system in the car to absorb the heat from the fluid that exchanges heat with the evaporator. Hence, the only things which were added are measurement probes and modifying refrigerant volume flow rate.
Third experiment shows an improved system by eliminating the cold fluid circuit as shown in figure 12. It is directly linked the intercooler outlet air with the evaporator of the A/C system (ICev). The main advantage of this setup is reducing the costs of weight, additional parts/accessories and space. Hence it would increase the efficiency of the cooling system by eliminating the losses due to the lining of the extra heat exchanger.
Figure 12: Test setup 2 with improved intercooler/evaporator system
Overall the test showed such modified A/C systems can improved efficiency by up to 9%; however, the second test setup was not experimented, but the author wanted to show that there are promising potential in upcoming automobile market through modifying existing passenger cars without having to redevelop new cars. 
2.5 Inlet Manifolds
Previous chapters were discussing the importance of lowering the temperature and humidity of the charged intake air in improving engine performance and lowering emissions considerably. This chapter will discuss about another factor that would enhance effectively engine performance through creating strong induction swirl inside the cylinder to enhance the fuel-air mixing quality. The typical properties of intake manifolds are to have; low resistance surface for airflow and typical lengths of runner and branch that take the advantage of tuning effects and ram.
Mergary et al.  found out that volumetric efficiency and air flow rate towards the cylinder are directly proportional to the fixed length of the intake duct at a speed of 1000-3000 rpm. Nowadays new technology has been introduced which is using the variable length induction manifold. As a result, enhance the torque conveyance at low speed without harming high speed power. In another words, when high rpm is needed, shorted length induction manifold is utilized while for low rpm and high torque, longer length induction manifold to be used.
Three major intake manifold designs are always under intensive studies; spiral, helical and helical-spiral manifold designs as shown in figure 13. Moreover, the interested output is always related to higher swirl intensity which will result in higher volumetric efficiency. Through researches, it was found that using helical-spiral manifold design result in higher engine performance and lower exhaust emissions compared to the other normal, spiral and helical manifold designs.