Archive for category b. Scientific background

The scientific background

This air gas hybrid will be built on the process ideas put forth by the Scuderi split cycle air gas hybrid motor, however, applied to a readily available, normal gasoline motor. The goal is to show that increased performance and efficiency can be achieved, without significant impact, by creating a hybrid engine system from what is already available. This project will be split into two main foci, converting one of the two cylinders in the motor to act as a compressed air source and developing a controllable system for storing and applying the stored air charge.

Before an in-depth discussion regarding the development of a new system can occur, one must first evaluate current technology and the basics of the internal combustion engine. An internal combustion engine is a complex gas power cycle where stored energy, in the form of chemical fuel, is transformed into kinetic energy through controlled combustion and allowed expansion. This expansion, via a crankshaft, becomes a useable force. However, combustion of a fuel is a non-cyclic process. The working fluid, an air-fuel mixture, undergoes a permanent chemical change and after use is discarded to be replaced with a new intake charge. For the purpose of thermodynamic evaluation, this gas power cycle must be represented as an air standard cycle. For calculation purposes, a mass of air operates in the complete thermodynamic cycle and heat is added and rejected via external reservoirs. This process is to be considered reversible. For the purpose of analysis, certain assumptions must be made. The working fluid is considered to behave as an idea gas with constant specific heats. The combustion process is replaced with heat addition; exhaust with heat rejection.

The following terms are to be defined for ease of discussion:
TDC; Top Dead Center: Position of the piston at top of stroke
BDC; Bottom Dead Center: Position of the piston at bottom of stroke
Stroke: Distance between TDC and BDC
Bore: Diameter of the piston
Compression ratio: ratio of maximum volume to minimum volume VBDC/VTDC
Engine displacement = (# of cylinders) x (stroke length) x (bore area)

MEP: mean effective pressure: A const. theoretical pressure that if acts on piston produces work same as that during an actual cycle
Wnet = MEP x Piston area x Stroke = MEP x displacement volume

Figure B1: The four stroke process

Figure B1 shows the basic four stroke internal combustion engine. This process consists of four distinct strokes, hence the name. During the intake stroke, the piston moves down in the cylinder and the intake valve opens. This draws a fresh fuel/air mixture into the cylinder. As the piston moves back up, the intake valve closes, and the air is compressed. This is the compression stroke. At the beginning of the power stroke, the now highly compressed fuel/air mixture is introduced to spark via an ignition source. This causes combustion, adding heat to the process. A rapid expansion of gasses applies pressure to the piston, forcing it down, converting the potential energy stored in the fuel to kinetic energy. After the power stroke, the exhaust valve opens and the piston returns to TDC, exhausting the combusted fuel/air mixture, finishing the exhaust stroke.

As earlier mentioned, for the purpose of analysis, this four stroke process is converted to an idealized air standard cycle. In this case, the Otto cycle is used, represented in Figure B2.

Figure B2: The Otto cycle

Process 1-2 is an isentropic compression. Work (Win) is applied via the crankshaft, compressing the air. If one applies the First Law of Thermodynamics;

U2-U1 = Q – Win
Q = 0 (since, reversible adiabatic compression)
Win = U2-U1

Process 2-3 is a constant volume heat addition;

U3-U2 = +Qin – W
W = 0 (since, it is a constant volume process)
Qin = U3-U2

Process 3-4 is an isentropic expansion;

U4-U3 = Q – Wout
Q = 0 (rev. adiabatic expansion)
Wout = U4-U3

Finally, process 4-1 is a constant volume heat removal;

U1-U4 = – Qout + W
W = 0 (no piston work)
Qout = U4-U1

The thermal efficiency is given by:

The specific heats are assumed to be constant.

Here y=1.4 at ambient temperature

Effectively, the efficiency of the internal combustion process can be increased through a rise in compression ratio.

The mass air flow through a 4 stroke motor is governed by the following equation:

where D is the displacement, N is the rotational velocity, and 120 is a factor of 60 to convert from revolutions per minute into second and 2 for the amount of crankshaft rotations per intake stroke. A motor that were to intake air every stroke would simply have 60 in the denominator.


Leave a comment