Needs Behind Variable Valve Timing

As engines are getting modern day by day scientists and engineers are planning to get more and more control over them. Such as they are computerizing and electrifying the engine, leading to reliable, fuel-efficient, better performance, and environment-friendly engines.

One way to get better control over engines is to control their valve timing and valve lift. This is done by control units, sensors, and actuators. The problem with continuous valve timing was that we can not optimize the engine’s power and performance regarding our needs. Let’s say there is a need for less air-fuel mixture during lower loads and RPMs, but with constant valve timing, it will input the same amount of air-fuel mixture when this mixture is required in higher amounts such as during higher loads. So, we can get more or less power by the number of rotations of an engine at a specific time or by gears, which is inefficient.

By Variable Valve Timing we can optimize this as we can reduce the amount of air/fuel mixture entering the engine during lower loads and increase this during higher loads. So, in this way, we can optimize the engine performance along with fuel consumption.

History

In August 1903, variable valve timing was first used in Cadillac Runabout and Tonneau invented by Alanson Partridge Brush. This car was a Horseless carriage which was powered by Single-Cylinder Engine. These engines were about 10hps of Power. These were among the first automobiles produced by Cadillac. They used variable-lift intake valve.

In these early days, Alfa Romeo also developed Variable Valve Timing with double overhead camshaft(DOHC) engines with different low- and high-speed operation profiles. This brand was the first to use Variable Valve Timing in 1980.

During the last years of the same decade, Japanese Companies like Nissan(NVCS) and Honda(VTEC) developed their engine with Variable Valve Timing. The First Honda engine with VTEC was B16A

Porsche Developed their Variable Valve Timing Technology in 1992 with the name of Variocam in Porsche 968.

Another German Automobile manufacturer BMW developed VANOS and Valvetronic as an answer to Variable Valve timing in 1992 and 2001 respectively.

How Variable Valve Timing had improved fuel efficiency and performance?

In this section, we will discuss the concept behind variable valve timing, some of its terminologies, and how it has an impact on engine performance & fuel efficiency.

Valve Timing Diagram

This diagram shows 360 degrees of movement of the piston from TDC to BDC. It also tells us when a specific valve opens or close.

Basic Terminologies of Variable Valve Timing

Valve Overlap: Valve Overlap occurs when both intake and exhaust valves are open. This period is about 10–20 degrees of rotation of the crankshaft. But the problem is that when both valves are open simultaneously, it may same as the air/fuel mixture loss through the exhaust port. To solve this problem, valves and engines are designed so that this loss won’t happen. Valve lift during valve overlap is also little. Moreover, this overlap should not last for too long.

You can see this following graph which gives us information about Valve Lift, Piston’s position and crankshaft rotation.

Valve overlap is really helpful at high speeds as it provides some extra air/fuel mixture Formula for Valve overlap is given Below:

Exhaust Valve Closing Point + Valve Opening Point = Valve Overlap

To understand this let’s take an example the intake valve opening point is 10 degrees BTDC(BTDC means Before Top Dead C) and the closing point of the exhaust valve is 10 degrees ATDC(After Top Dead Center) then valves will overlap for 20 degrees rotation of the crankshaft.

Valve Lead: This terminology refers to the opening of valves before TDC or BDC. As the intake valve opens before the piston reaches TDC similarly exhaust valve opens before the piston reaches BDC. The valve leading to the intake valve ensures more fresh air to the combustion chamber as it increases the time to input more air into the combustion chamber. While for exhaust valve it improves efficiency to pump out more exhaust gasses. This also lowers pumping losses.

Just add 180 degrees to the opening and closing points to calculate the intake valve timing event duration. Such as an inlet valve opens at 12 degrees BTDC and closes at 30 degrees ABDC so the intake valve timing event would be:

180 + 12 + 30 = 222 degrees.

This means the intake valve remains open for 222 degrees of crankshaft rotation.

Valve Lag refers to the inlet valve closing after the piston reaches BDC while the exhaust valve closes after the piston reaches TDC. These happen at the beginning of compression and suction/intake stroke respectively.

The following diagram will help you to understand different strokes in 4-stroke petrol engine

Intake Stroke: In the Constant valve, when the engine cycle starts it opens the inlet valve at TDC but for variable valve timing the inlet valve opens 10–20 degrees in general before the piston reaches TDC. The inlet valve closes 25–35 degrees after the piston reaches BDC. This gives some extra time within an inlet stroke to input more fresh air/fuel mixture.

Combustion Stroke: The compression starts after the inlet or intake stroke when the piston moves to TDC from BDC. As you know that at the end of the compression stroke spark ignites the air/fuel mixture. In the case of variable valve timing, this happens 25–30 degrees before the piston reaches TDC, this gives more time to burn the air/fuel mixture and properly propagates flame which further optimizes the engine’s performance.

Expansion or Power Stroke: This stroke starts after the combustion occurs which applies and pushes the piston to move downward to BDC. This stroke provides power to the engine, so it is called a power stroke. This stroke ends when the exhaust valve opens 50 degrees before the piston reaches BDC.

Exhaust Stroke: This stroke starts when the exhaust valve opens exhaust gases are output from the exhaust port. This stroke starts 30-50 degrees before the piston reaches BDC during the expansion stroke and ends after 10-20 degrees of the piston reaching TDC.

Valve Lift: Valve lift refers to how much a valve moves upward or opens when required. Valve lift is low at low engine speeds and is greater at high engine speeds.

Cam advance refers to valve opening and closing earlier. This happens to both intake and exhaust valves. Such as the intake valve opening when the piston approaches the Top Dead Center or TDC. Also, it ignites the air/fuel mixture earlier. So, if an engine is designed to open the intake valve 25 degrees before TDC, a 5-degree cam advance means it will open the intake valve 30 degrees earlier to TDC. The same will happen for the closing of the intake valve. You can use the following image to understand it in a better way.

Note: Remember that the duration of valve opening remains the same as if an engine camshaft is designed to open the intake valve for 212 degrees, it will remain open for 212 degrees of the rotation of the crankshaft.

Cam Retard: Cam retard delays the opening and closing of both intake and exhaust valves. This means if intake valve opens 25 degrees before Top Dead Center or TDC during normal cam timing, 5 degrees retard means it will open 20 degrees before TDC.

 This has a lot of benefits like reducing knock at lower engine speeds, smooth idle, provides more power during high RPMs and fuel economy. Cam retard is also helpful when there is too much low end torque() and you loss traction.

Benefits of Variable Valve Timing

The main benefits of variable valve timing are that it reduces fuel consumption and optimizes power and torque output. As it includes two types of technologies variable valve lift and variable valve timing. So we have more control over the engine.