Investigation of fuel economy of gas turbine in hybrid cars

Investigation of fuel economy of gas turbine

in hybrid cars  

M. Ben Chaim*, M. Brand* 

(*Full Professor of Mechanical – Mechatronics Department, faculty of

Engineering – Ariel University Center of Samaria, Ariel, 40700, Israel) 

O. Gelashvili*, N. Navadze** 

(*Full Professor, **Associated Professor, Georgian Technical University,

77, M. Kostava str., 0175, Tbilisi, Georgia) 

Abstract: Though since recently, hybrid cars are having acceptable technical parameters. Until now, hybrid cars have been basically using traditional internal combustion engines, either working on diesel or gasoline, due to their state-of-the art nature and relatively high thermal efficiency. However, when analyzing engine operation modes in hybrid cars, and more particularly, their ability to operate in an optimal mode, a gas turbine usually comes to mind, which is comparable to other thermal engines due to its exceptional technical properties. Determining its performance in hybrid cars in terms of fuel economy alone is technically incorrect, since all aspects of this technical solution shall be taken into account. This paper is an attempt to evaluate the performance of gas turbines as a main source of energy in hybrid cars by means of system analysis.  

Keywords: hybrid car; gas turbine; fuel economy; high efficiency; car performance.  

1. INTRODUCTION

 

      The idea of using gas turbine engines in cars is fairly old. However, its design has just recently achieved perfection that enables to implement those both in vehicles and in fixed objects. State-of-the-art blade engines technology, metallurgy, composite materials, and production technologies allow development of reliable gas turbine engines, which are perfectly capable to replace piston combustion engines in cars. Gas turbine engines feature a lot of advantages over traditional piston engines. Though it still has its flaws, these can be handled, being gradually corrected during construction design. While talking about a gas turbine, it should be noted that it is capable of developing high speeds. This provides for a significant improvement of weight parameters of the engine itself, and also of the generator (alternator) used to charge batteries. Eventually, this significantly reduces the total weight of such power plant. Based on the review of available literature sources, the following main advantages of gas turbines are worth mentioning [1-9].

1. Low specific volume per power unit;
2. Low specific weight per power unit;
3. Lower operation costs;
4. Lower mean time to overhaul;
5. Good environmental indicators;
6. Parts range reduction;
7. Simple adjustment to other types of fuel;
8. Great reduction in generator weight and volume due to high shaft rotation speed.

At this stage of development and production, gas turbine has the following flaws:

1. Low thermal efficiency;

2. High initial cost.

     

2. Basic part

 

    To enable comparative evaluation, calculations have been made for a 9.800 N minicar. We will deal with a car moving in general conditions, with acceleration occurring on gradient–free roads alone. Assume that a car accelerates to , i.e., the car speeds up to 100 km/h within 10 seconds, like most cars in this class. Required energy will be determined for a mixed cycle, which is both urban and non-urban traffic.

    In our calculations, we were governed by Directives of UN ECE [9]. Currently, two major sets of regulations are applied in Europe, which are binding upon all car producers. These are EU Directives, and norms and regulations of UN Economic Commission for Europe applicable in most countries worldwide (UN ECE). Fuel consumption is normally indicated in three traffic modes: urban, non-urban, and mixed traffic. According to UN ECE requirements and above Directives, tests are conducted on chassis dynamometer workbench.

    According to these rules and regulations, the following speeds were assumed: maximum urban speed is 50 km/h; average urban speed is 19 km/h, provided that the car speeds up every half kilometer. Required acceleration energy was determined from initial to maximum speed, which is 50 km/h. As regards non-urban traffic, maximum speed is 120 km/h; average speed is 63 km/h, provided that the car speeds up from 63 km/h to 120 km/h per each 5^th kilometer. Mixed traffic indicators were assumed as an average of fuel consumption for urban and non-urban traffic.

    Considering that thermal, electrical and effective efficiency and specific fuel consumptions rate are applied in technical sources to compare thermal engines performance, with a major gap existing between the results, we would rather use both indicators in our calculations, i.e., electrical or effective efficiency, and specific fuel consumptions.

    Fuel consumption is determined in car theory based on specific fuel consumptions [1, 10], while assuming that the car is in a continuous acceleration mode, i.e., the required engine maximum power is determined, with fuel consumptions rate per 100 km calculated afterwards based on the results. Though such approach is fairly adequate in traditional cars, it is absolutely unfit for hybrid cars due to their technical solution, namely: hybrid car engine operates in an optimal mode; and the car uses acceleration energy of rechargeable batteries. Considering the above reasons, and fuel economy assessment technologies, as stated in EU Directives [9] and in norms and regulations of UN Economic Commission for Europe [9], we have obtained and used new fuel consumption equations.

    Fuel consumption in terms of specific fuel consumptions is determined by the following equation:

, L/100km,                                                                     (1)

where is specific fuel consumptions, g/kWh; is car weight, ; is car power at cruise speed, kW; is fuel density, kg/l; is the number of accelerations per 100 km, which differs for urban and non-urban traffic modes; is car speed before acceleration, ; is car speed after acceleration ; is free fall acceleration, ; is unbalanced mass ratio of cars and engines.

        Car power at cruise speed is:

  KW,                                                                                          (2)

  where wheel power at cruise is speed, kW; is a power required to overcome the resistance of road or wheel rolling motion, kW; is a power required to overcome air resistance, kW. These powers are determined by the following equations:

               

                ,

where is wheel rolling resistance ratio at a low speed; V_a is speed, ; K is aerodynamic resistance coefficient, , is cars frontal area, .

We obtain therefore:

1. Urban traffic:

,   L/100km.                                 (3)

2. Non-urban traffic:

,  L/100km.                                 (4)

    Fuel consumption by effective efficiency is determined by the following equation:

 , L/100km,                                                                                    (5)

where  is fuel consumption, Liter per 100 km;  is car movement energy per 100 km, kWh; is engine effective efficiency; is car transmission efficiency; is engines thermal efficiency; is engines mechanical efficiency;   is 1 liter of fuel heat output, kWh/L.

    The following equations were obtained to determine :

1. Urban traffic:

 KWh                 (6)

2. Non-urban traffic:

   KWh              (7)

where   is car weight, ; is wheel rolling resistance ratio at a low speed;  is speed, ; is Aerodynamic resistance coefficient, ;  is cars frontal area, ; is free fall acceleration, ; is unbalanced mass ratio of car and engine.

    As seen from equations (1), (3), (4), (6) and (7), total car weight is essential to compare hybrid car fuel consumption parameters for various thermal engines. Table 1 summarizes engine performance, as taken from various literature sources [1-4, 7, 8, 11 and 16].

Table 1.

Techno- economic performance of engines

 

    A gas turbine-equipped car is more lightweight due to a low specific weight of gas turbine and associated generator. Weight relationship for various cars was determined by means of Table 1 and equation (8) used in [8] to calculate the weight of generator (alternator), wherein the mass of generator is determined as follows:

,                                                                                              (8)

where is engine power, W;  is engine starting time, s; is rotor radius, m; is engine axial speed, rev/sec ( ).

    Results are summarized in Table 2.      

            Table 2.

Weights of cars with various engines

 

     Fuel consumption rates are summarized in Table 3 and Figure1.

Table 3.

Various engine hybrid cars fuel consumption for urban traffic in terms

  Of specific fuel consumptions  

 

   Fig.1. various engine hybrid cars fuel consumption (mpg) for urban traffic

    Fuel consumption rates for various cars are specified below for the sake of comparison [7, 8, 12, 13, 14 and 15].

      Table 4.