MBD Simulation on IC Engine Valve Train

Aim:

Modelling and analysis of IC Engine Valvetrain for detecting the valve displacement and contact forces between contacts.

Objectives:

Obtain valve lift for given CAM lift of 3.5 mm & 6 mm at the same speed (1500 RPM)

Obtain Plots between contact force of Cam & Push rod, Push rod & rocker arm, and Rocker arm & valve respectively

Working Principle

The primary functions of the valvetrain are to control the opening and closing of the valves. valvetrain also controls the flow of air and fuel that enters the combustion chamber and leave as exhaust gases.

In the working of a valvetrain, the camshaft plays a very important role as its rotational movement aid the opening and closing of the valves using the cam lobes. Although various parts help the valve train to work.

In most heavy-duty diesel engines, four valves are used, that is, there are four valves in each cylinder. Two valves for the intake air/fuel and two for the exhaust gases. The intake valves have a larger diameter than the exhaust valves, which allows much airflow to the cylinder.

Exhaust valves are designed to withstand higher temperatures of hot exhaust gas than the intake valves. This is because the fresh air that flows through the intake valves keep it at a lower temperature.

To keep the valves intact, that’s from burning, the intake and exhaust valve transfers heat to the cylinder head else they get burnt.

There is an injector in the middle of the valve which is pushed down to inject fuel into the cylinder. Well, the timing for the valve opening and closing is incredibly precise, making it’s working efficiently.

In newer engines electrical signals are used to nod the injector, instead of the mechanical valve train. This process is even more accurate.

Components of a valve train

Below are the different components that aid the working of a valve train:

Camshaft:

The function of the camshaft is to control the timing and lift the profile of the valve opening. This is achieved with the cam lobe on the rotating shaft.

A camshaft is driven by the crankshaft and it rotates at half speed of the crankshaft in the case of a four-stroke engine.

The crankshaft transfers motion to the camshaft using a metal timing chain or mostly a rubber timing belt. A set of gears can also be used.

Pushrod:

A pushrod is a long, slender metal rod that is used in overhead valve engines. It’s also used to transfer motion from the camshaft in the engine block to the valves in the cylinder head.

There is a lifter on the bottom end of a pushrod that gets in contact with the camshaft. Camshafts lobe moves the lifter upwards, which moves the pushrod. The top end of the lifter further pushes on the rocker arm, which opens the valve.

Rocker arm/bucket tappet:

The engine design determines the ways the valve will be actuated, either by a rocker arm, finger or bucket tappet. In an overhead camshaft engines bucket tappets or fingers are used, upon which the cam lobes contact.

Rocker arms are used in overhead valve engines, which are actuated by a pushrod and pivot on a shaft. It can also be pivoted on individual ball studs which will help to actuate the valves.

Valves:

The valve used in most modern engines is called a poppet valve. Some valves like sleeve valves, slide valves and rotary valves are also considered at times.

Generally, poppet valves are opened by the camshaft lobe or rocker arm. It’s closed by a coiled spring called a valve spring.

IC Engine Value Train Mechanism

Modelling of IC Engine value train parts

Rocker

Pushrod

Cam

Valve mount

Engine valve

Assembly view of IC engine valve train

Motion Analysis for cam

Case 1: Cam lift distance 3.5mm

To calculate the distance between the two centres(L)

Given: R1 = 12.5; R2 = 5; Cam lift = 3.5mm

Cam lift = (L-R1)+R2

L = (12.5+3.5) - 5

L = 11mm

With the above information, Cam is modelled for the case 1 analysis.

Case 2: Cam lift distance 6mm

To calculate the distance between the two centres(L)

Given: R1 = 12.5; R2 = 5; Cam lift = 3.5mm

Cam lift = (L-R1)+R2

L = (12.5+6) - 5

L = 13.5mm

With the above information, Cam is modelled for the case 2 analysis.

Motion analysis:

1. Establish contact between Cam and push rod, pushrod and rocker arm, and rocker arm and valve.
2. provided the rotary motion to the cam with 1500 RPM.
3. Set up a linear spring between the valve and the valve stem(as seen in the assembly) with the spring stiffness of 10 N/mm and a free length of 45 mm.
4. Change frames to 7200 frames per sec and use precise contact.
5. Select the entire assembly and assign the material Carbon Steel.
6. Select the required plots for results and analysis.
7. Simulate for the required time.

Results

Case 1

The maximum displacement of the valve is 8mm

the maximum friction force is 863 N in between the Engine valve and pushrod

the maximum friction force is 694 N in between the Rocker and pushrod

the maximum friction force is 233 N in between the Cam and Rocker

the maximum and minimum friction force are 57 N and -52 respectively in between the Cam and Rocker

the negative force due to the spring opposes the motion of the valve and rocker

Case 2 plots

the maximum displacement of the valve is 14mm

the maximum friction force is 738 N in between the Cam and Rocker

the maximum friction force is 2107 N in between the Pushrod and Engine valve

the maximum friction force is 2306 N in between Pushrod and Rocker

the maximum and minimum friction force are 179 N and -70 respectively in between the Cam and Rocker along x-direction

the negative force due to the spring opposes the motion of the valve and rocker

Observation:

The contact forces are gradually reduced from the cam to the valve in both cases.

The contact force between rocker and valve along X direction experience negative due to the spring opposes the motion of the valve.

Conclusion:

Thus, the modelling of the IC Engine valve train mechanism is done and the motion analysis is carried out to get some useful plots for further analysis.