Valve Lift Analysis of a FlowBench

FlowBench:

An air flow bench is a device used for testing the internal aerodynamic qualities of an engine component and is related to the more familiar wind tunnel.

It is used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines. It is also used to test the flow capabilities of any component such as air filters, carburetors, manifolds or any other part that is required to flow gas. A flow bench is one of the primary tools of high performance engine builders, and porting cylinder heads would be strictly hit or miss without it.

Introduction:

Homogeneous air-fuel mixture that enters the combustion chamber during intake process can tremendously optimize the engine performance output. There are a number of factors that affect the mixture, but this study focuses on the flow through a cylinder head port. Since flow losses are caused by port expansion rather than contraction, the greatest flow loss in the intake port is due to expansion of air out of the valve.

The air flow through the engine is directly controlled by the valve lift, so greater flow should occur at the maximum valve lift atleast up to a certain point. The experiment covers the intake value of flow rate due to the valve lift.

The design of the cylinder head for automotive SI engine is important because it affects the performance, fuel economy and emissions. Designing and developing an effective intake port is one of the major research areas in SI engine, since it determines the volumetric efficiency and in-cylinder charge motion of the spark ignition engine, which influences the thermodynamic properties and is directly related to power output and emissions. In order to achieve higher thermal efficiency and better emission control, intake air motion should be enhanced in combustion. An interesting challenge in designing the SI engine intake port is to pursue a maximized mass flow while maintaining in-cylinder motion within a reasonable range. The volumetric efficiency can be improved by allowing more mass flow into the engine. Delivery of the air-fuel mixture to the combustion chamber is one of the most important aspects in designing effective cylinder head. Loss of horsepower and torque in the operating range of the engine due to inappropriate cylinder head port geometry causes the air-fuel mixture to drip out of suspension or puddle on the floor of the port.

Turbulence intensity, swirl and tumble content in the incoming airflow to the combustion chambers are the key parameters in controlling the emission level. The postulations of several flow regimes are classified by the occurrence of flow separation at the valve face or valve seat. The separation at the valve face and seat has been shown to be crucial for the prediction of flow losses in intake regions. There could be little variation of the flow with valve lift in the upstream part of the port, but ‘the flow pattern in the vicinity of the valve and inside the cylinder may vary with lift’.

Port and valve design determines the discharge coefficient (Cd) of the port and consequently the volumetric efficiency of the engine. Discharge coefficient describes the behavior of all real flows contract in area as they pass through any restriction. Since the performance of the engine is dependent on the amount of air entering the combustion chamber, the port design introduced in this study is aimed to increase the flow and improve its quality.

Valve Design:

As the airflow passes various components and stages of the intake system, different properties and characteristic of the intake charge have been modified to achieve the overall goals of the intake charge management system. The intake air filter ensures that air cleanliness is adequate, the charge air composition and oxygen content is controlled by introducing EGR to the intake air and the compressor and charge air cooler ensure that intake manifold pressure and temperature objectives are met and that intake charge density is within design limits.

Valves or ports control the timing of air flow to the cylinder. Also, the passage between the intake manifold and cylinder can have a significant influence on the flow as it enters the cylinder and can be used to impart a suitable bulk motion and kinetic energy to the charge to support the mixing of air, fuel and intermediate combustion products in-cylinder.

In four stroke engines, intake gas enters the cylinder through a port located in the cylinder head and past a valve used to open and close the port. In two stroke engines ports in the cylinder liner that are alternately covered and uncovered by the piston are commonly used.

Gas flow into and out of the cylinder in 4-stroke engines is controlled almost exclusively by poppet-style valves. While other valve designs have been used or proposed, none appear to be able to match the reliability and sealing ability of the poppet style valve. The most common poppet valve construction in automotive use is the one-piece valve where the entire valve is made from the same material. However, other variations are available including:

• A welded tip construction has a separate tip welded to the stem above the keeper grove. The tip can be made from a material that is much more wear resistant than the rest of the valve.
• A two-piece construction has a separate stem welded above the fillet, Figure 2 left.
• An internally cooled construction has a hollow stem containing a coolant such as metallic sodium or sodium-potassium mixture and is commonly used in extreme duty and high performance exhaust valves, Figure 2 center. Valve temperature peaks are reduced due to the “shaker effect” of the molten metal and these designs can withstand thermal loads particularly well. The temperature in the hollow neck can be lowered by about 80 to 130 K, reducing overall wear of the valve and valve seat insert.
• Some designs also have a hollow cavity in the valve head that contains metallic sodium, Figure 2, right. This is an extension of the classic sodium-filled hollow valve, with an additional cavity in the valve head. This can further temperature peaks in the valve head and further increase the valve service life.
• A welded seat face construction has a valve seat that is welded with a hard overlay to better withstand conditions that would otherwise lead to extreme valve seat wear and/or corrosion.

Figure 2. Example poppet valve designs

Left: Two-piece solid stem valve. Center: Hollow stem valve.
Right: Hollow stem valve with additional cavity on the valve head.

FlowBench Design:

An interesting challenge in designing the SI engine intake port is to pursue a maximized mass flow rate while maintaining the in-cylinder motion within a reasonable range. So for that we need to increase the volumetric efficiency of the engine i.e. the breathing capacity of an engine.

In in-cylinder, fluid motion in an internal combustion engine is one of the most important factors controlling the combustion process. As the flow takes place inside the cylinder, the turbulence intensity is also increased.

Turbulence is something which is important for the proper mixing of air and fuel inside the combustion chamber. The inlet manifold configuration affects the turbulence of the fluid inside the cylinder. The turbulence inside the combustion chamber is recognized in two ways: 1. Swirling Motion; 2. Tumble Flow.

Swirl Flow: It is also known as the axis symmetric flow. Here the conservation of angular momentum results in the creation of a free vortex flow in which circumferential velocity increases as the radius decrease and then decays to zero at r = 0 due to actin of viscosity.

The strength of the swirl flow is calculated by the swirl ratio, a dimensionless parameter, a ratio of angular velocity to axial velocity.

Tumble flow: it is formed about the circumferential axis near the edge of the clearance volume in the piston crown or in the cylinder head, which is caused by squishing of the in-cylinder volume as piston reaches near TDC. It is also known as vertical swirl or barrel swirl.

During the compression stroke, swirl flow interacts with the tumble flow forming a squish air flow motion.

Boundary Conditions:

Inlet: Total Pressure = 2.1 bar

Outlet: Static Pressure = 1.01 bar

Level of Mesh: 6

Grid Dependence Test:

It involves running simulations on successively finer mesh sizes to determine the effect of the mesh size on the accuracy of the result. Generally 3 mesh sizes are chosen – Course, Medium and Fine. These results are then compared to check the variation on values. The computed results shows that we get more finer results as increases the level of mesh. At a certain point, it is observed that the results do not change, so thus further refinement of mesh will only lead to waste of time and increase in run time.

The Grid Dependence test is carried out for 4 basic mesh - 3, 4, 5 and 6 respectively and the results were recorded accordingly.

Summary	Design Point 1	Design Point 2	Design Point 3	Design Point 4
Level of initial mesh (Automatic Mesh)	3	4	5	6
SG Mass Flow Rate 1 [kg/s]	0.081501406	0.068257732	0.065346021	0.063483309
SG Max Velocity 1 [m/s]	101.7123851	82.89948217	78.78569819	76.27149728

It is observed that as we make the mesh more refine, we start getting fairly more accurate results. We observe that at mesh 5 and 6, the results seems to be converging. There is a very minute change in the mass flow rate and maximum velocity.

So, in order to get a more accurate results, we continue our flow simulation with mesh size = 6.

Observations:

Summary	Design Point 1	Design Point 2	Design Point 3	Design Point 4	Design Point 5	Design Point 6
Valve Lift (m)	0.003	0.004	0.005	0.006	0.007	0.008
SG Mass Flow Rate 1 [kg/s]	0.04340967	0.061655309	0.104759926	0.122480674	0.133172605	0.140110028

The table clearly shows that the as the valve lift increases, the mass flow rate of the fluid into the cylinder also increases. As Valve Lift is the distance by which the valve is raised from the seated position. The camshaft controls the speed of the valve opening. The maximum allowable valve lift is of 9.9mm only.

As the mass flow rate increases, the velocity of the fluid at the inlet also increases which results in decrease in the swirl velocity and increases the discharge coefficient. This exhibits a better engine performance results.

Results for seperate Valve Lift:

Valve Lift: 0.003m

Velocity:

Pressure:

Valve Lift: 0.004m

Velocity:

Pressure:

Valve Lift: 0.005m

Velocity:

Pressure:

Valve Lift: 0.006m

Velocity:

Pressure:

Valve Lift: 0.007m

Velocity:

Pressure:

Valve Lift: 0.008m

Velocity:

Pressure:

Graph Between Mass Flow Rate VS Valve Lift

Results:

1. As the valve lift increases, the mass flow rate also increases.

2. The increase in mass flow rate leads to increase in velocity of the fluid at the inlet, thus decreeae in pressure, satisfying the Bernoulli's Theorem.

3. The swirl and tumble ratio decreases with increase in valve lift.

4. The flow velocity at region between the valve seat and the port keeps on varying as the valve lit increases. At first, the fluid velocity is maximum between the valve seat and port for valve lift of 0.003m, as it is quite obvoiu that the flow area is very small, satisfying the continuity equation. Now, as the valve lift increases, the velocity at that region decreases as the area between them increases.