- [Narrator] 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:
its valvetrain.
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 valvetrain,
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's cylinders need intake valves
that open to pull air in,
and exhaust valves that open to push the exhaust gases 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 crankshaft,
the valves stay in time with the rest of the engine,
regardless of RPM.
Valvetrain 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 pushrods and rocker arms
to actuate the overhead valves.
So, have you ever seen a vehicle
with a "sixteen-valve" or "DOHC" badge
and wondered what that meant?
Well, "sixteen-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 valvetrain layout -
two camshafts, positioned above the valves.
So that's how a valvetrain works,
but it's not the whole story.
Timing those valves correctly is a delicate tightrope 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 rpm
might require a different timing than at 5500 rpm.
That's where variable valve timing comes in.
Nearly all auto manufacturers
have a variation of this technology,
and Toyota's 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 RPM 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 Otto 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 flipside, however,
is it means that the pistons aren't firing
with the maximum possible amount of pressure and power.
This tradeoff 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 VVT-iW system,
where the engine can flip
between Otto and Atkinson modes as needed.
It also makes sense to use Atkinson 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 powertrain.
So that's a brief look at the valvetrain 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.