Engines 101: How Does a Valvetrain Work? | Toyota

In this installment of Toyota's engines 101 series,

we're taking a closer look at one of the most critical parts of an engine.

It's valve drain.

This component has a massive impact on the overall performance

and character of an engine since in a nutshell,

it's how the engine breathes.

So let's first explain what exactly constitutes a valve

train before getting into some more advanced aspects like

how valve timing works and how it can be

used to enhance not just performance but also efficiency

without a valve system.

The entire concept of an internal combustion engine simply wouldn't work.

An engine cylinders need intake valves that open to pull air in

and exhaust valves that open to push the exhaust gasses out,

effectively inhaling and exhaling with each cycle,

creating the power that ultimately turns the wheels.

These valves are opened and closed by a component called a camshaft.

By aligning the camshaft's lopsided lobes with the valves,

it can push against them as the camshaft turns opening each one with every rotation.

And since this camshaft is connected to the crank shaft,

the valves stay in time with the rest of the engine regardless of R PM.

Valve train design can vary in a few common ways.

How many valves there are,

how many camshafts operate them

and where the components are located.

First,

most engines nowadays use two valves on the intake side and

two valves for the exhaust making for four per cylinder.

Next, some vehicles use a single camshaft to manage both intake and exhaust valves.

While others use a dual camshaft layout with one for each side.

Finally,

while most modern vehicles have the camshafts mounted

above the valves to simply control them directly.

Some also place the camshaft below the valves in the block and instead use

a system of push rods and rocker arms to actuate the overhead valves.

So, have you ever seen a vehicle with a 16 valve or dohc badge?

And wondered what that meant?

Well, 16 valve on a four

cylinder engine indicates that each cylinder has four valves

and dohc stands for dual overhead cam,

which further describes that vehicle's valve train layout,

two camshafts positioned above the valves.

So that's how a valve train works, but it's not the whole story

timing.

Those valves correctly is a delicate tight rope walk between smoothness,

efficiency and power.

We're talking fractions of a millisecond making a huge difference.

That's why hitting an ideal balance at 1500 R PM

might require a different timing than at 5500 R PM.

That's where variable valve timing comes in.

Nearly all auto manufacturers have a variation of this technology.

And Toyota is known as VVT I or variable valve timing with intelligence.

The details of this feature can vary by model but the goal is the same.

The basic design centers around the cam gear

that pairs the camshaft to the crankshaft.

By implementing an internal shifting mechanism within this gear.

The rotation of the camshaft can be pushed slightly forward or backward

which then advances or delays the opening of the valves accordingly.

This way,

valve timing can be adjusted without impacting the rest of the engine cycle,

which in turn enables the ability to maintain

an ideal balance across multiple R PM ranges.

Now, this brings us to our next point

instead of targeting a perfect balance.

What if valve timing could be adjusted to maximize efficiency?

Well,

this is exactly the thinking behind Toyota's modern

take on the Atkinson cycle engine design.

This design takes a different approach to combustion compared with the usual auto

cycle engine of a typical vehicle

without taking a deep dive into automotive history. The general idea is this

with the Atkinson design,

the intake valves are kept open into the compression stroke,

this effectively shortens that stroke while still maintaining

an expansion ratio based on the full cylinder.

This in turn results in a lower compression ratio,

reducing the energy needed to compress the air fuel mixture.

The benefit here is that virtually all of the fuel is burned while

still inside the combustion chamber helping

to reduce emissions and maximize efficiency.

The flip side, however, is,

it means that the pistons aren't firing with

the maximum possible amount of pressure and power.

This trade off in power is why Toyota is selective about when it gets used.

For instance,

it makes sense to use it when full power isn't needed like highway cruising.

This is the idea behind Toyota's wide range VVTIW system where

the engine can flip between auto and Atkinson modes as needed.

It also makes sense to use Atkinson's cycle full time if it's

being supported by an additional power source like an electric motor.

This is the thinking behind a typical Toyota hybrid power drain.

So that's a brief look at the valve train design of an internal combustion engine,

not just what it is and how it works,

but also how its timing can be adjusted for either an optimum balance

of power and efficiency or even to crank the efficiency to the max

to learn more about the technology inside Toyota

vehicles or even just automotive engineering in general.

Be sure to check out the other videos in this series,

which can be found on Toyota's youtube channel.