Modeling and Control of Internal Combustion Engines M - Appunti e Domande d'esame

Alma Mater Studiorum - Università di Bologna

Laurea Magistrale in Ingegneria Meccanica

Laurea Magistrale in Advanced Automotive Engineering

Appunti Completi - Domande d’esame

Modeling and control of internal combustion engines M

Prof. Ing. Nicolò Cavina

Anno Accademico 2021-2022

Alma Mater Studiorum - Università di Bologna

Laurea Magistrale in Ingegneria Meccanica Laurea Magistrale in Advanced Automotive Engineering

Appunti Completi - Domande d'esame

Modeling and control of internal combustion engines M

Prof. Ing. Nicolò Cavina

Anno Accademico 2021-2022

Evolution of I.C.E. control systems

Def: Lambda parameter (i.e. normalized air-to-fuel ratio)

λ = (M_air/M_fuel) / (M_air/M_fuel)_stoich

Where:

(M_air/M_fuel)_stoich = 14.56

λ = 1 "Stoichiometric" Mixture: there’s the exact quantity of air needed to fully oxidize the fuel.

λ > 1 "Lean" Mixture: there’s an abundance of air w.r.t. the fuel to be oxidized.

Combustion conditions for different types of I.C.E.

S.I. engines (Spark Ignition)

1. The mixture is to be homogeneous; λ is the same in the whole combustion chamber.
2. Combustion takes place in a narrow window of λ values averaging more or less at the stoichiometric: λ ≍ 0.7 – 1.3.
3. In order to achieve a stable combustion a slightly richer mixture is preferable.

Compression ignited engine

The combustion is initiated by specific thermodynamic condition are reached to let the mixture self-ignite.

1. The mixture in the combustion chamber is non-homogeneous since the fuel is directly injected in the cylinder (the λ-value needed for combustion is reached locally and there is where the flame front begins).
2. Combustion takes place in a wider window of λ-values.
3. In order to reduce excessive particulate matter production the combustion happens in lean conditions: λ > 1.

Mechanical control systems for I.C.E.: The carburetor

Fuel path main trunk

• Throttle Valve (Butterfly Valve After Venturi)
• Venturi Duct
• Fuel Injection Tube
• Choke Valve (Butterfly Valve Before Venturi)

The first king of control systems of ICE were exclusively mechanical and based on the carburetor.

The carburetor is responsible for supplying and regulating the necessary quantity of fuel for a given air mass flow. Air is released in the carburetor by the partial vacuum created in the cylinders during the intake stroke (in the case of a “naturade engine). The flow passes through a venturi duct where it is delevated creating a pressure drop. This depression recalls liquid fuel from a reservoir injecting and mixing it in the flow of air.

In order to regulate the mass of fuel the carburetor’s architecture tables regulation the mass of air. This is responsible for the mixture of the pressure drop created in the venturi. This regulation is carried out in 2 different butterfly valves:

• The “Throttle Valve” is downstream of the venturi and allows less air (fuel mist) to get into the intake manifold/cylinders by creating an obstruction (i.e. depressive mode).
• “Choke Valve” is upstream of the venturi and to lower by the regulation the strat. While is not used to operate a biogas engine but to

What are the issues of a mechanical control for I.C.E.?

The issue of mechanical control systems for ICE is their inherent lack of flexibility.

The main problems of carburettors relate to:

1. Cold Start: In cold start condition the intake manifold is still relatively cold and therefore the fuel injected in the stream of air by the carburetor has an hard time evaporating and thus enter in the combustion chamber. In fact, fuel deposits form in liquid form on the cold walls of the runners. This brings the mixture entering the C.C. way above normal run expected, possibly exceeding the far-right λ-value acceptable for a stable combustion (λ << 1). This would result in misfire and thus loss of torque, with increased emissions.

   In order to avoid this a larger quantity of fuel is to be injected in the airflows in order to compensate for the fuel deposits on the walls. This can be done by acting on the upstream butterfly valve (“choke valve”) that by being closed increases the depression at the throat of the venturi thereby recalling more fuel from the reservoir.

   * C085: A common issue arises from the use of the choke valve. In fact, due to the temperature of the running valves as high around the cold fuel rich deposition will not evaporate getting into the cylinders and thus creating a richer mixture than the one wanted. This results in a higher % of non combusted hydrocarbons and possibly too rich of a mix for stable combustion (λ < 0.7), an overall far