Hello everyone and welcome
Perhaps you have driven or perhaps you've purchased the vehicle with a small turbocharged engine and it claimed you know
It's going to have the power of a v6 and yet the fuel economy of a small four-cylinder
and then you're actually driving around and you're noticing your numbers don't quite match up with the numbers you saw on the sticker saying you're
Gonna get this kind of fuel economy in this car
so in this video
We're gonna be talking about why small turbocharged engines may not always get the great fuel economy
That they claim to and so we're gonna talk about some of the benefits
There are legitimate reasons why you might want to have a downsized turbocharged engine
But then we're going to get into why you may not actually see in the real world those kind of fuel economy numbers
That the manufacturers are claiming it will get and so starting off with the benefits
Why might you downsize an engine and throw a turbocharger on it? Well, the benefits are very real the engines smaller it weighs less
It's got less moving parts. You've got less friction. You've got less pumping losses
And so depending on the loading scenario on that engine how you're driving it you can actually get really good fuel economy with these small
Turbocharged engines, but then you start asking for power and things change up a bit and so looking at an engine here
And I've got plenty of videos talking about how turbochargers work, but a very simple representation here. We've got our engine
We've got the turbocharger this thing spools up with the exhaust gasses and it pulls in additional air
So let's say you know
We have atmospheric pressure within this engine that's about fourteen point seven or about fifteen psi. And
Using this turbocharger this compressor right here. We're able to add in seven-and-a-half psi of
Additional boost so now we've got about 50% more oxygen within this engine and as a result
We inject more fuel and as a result of having more air and more fuel burning we get you know
Somewhere around 50% more power. So the idea behind a turbocharger is very clever
It allows you to have small displacement engines that still get a lot of power
But it may come down to being power or efficiency
Not necessarily, you know both at the same time, even though there are ways to indeed have both at the same time
Which we'll get into later. Now. The problem with turbocharged engines is knuck
And so one of the things manufacturers have to design for is how do we eliminate not?
so what is not well as that piston is compressing that air & fuel mixture on its way up towards the top of the
Cylinder, you have your spark plug fire and it of course ignites the air fuel mixture
But the temperature and the pressure because it's turbocharged is very high
and so you may have
pockets where that air fuel mixture
Is hot enough that it ignites on its own and then as a result
You have two ignition sources
You have these
colliding flame fronts and in a worst case you can have quite severe damage if you have really bad engine nook and so a
Turbocharged engine it's adding in air. It's adding in fuel and
As a result you're having higher cylinder pressures
So each individual cylinder has higher pressures and as a result
Each individual cylinder has higher temperatures and all of this gives you a greater likelihood to have not okay
so what things can the
manufacturers do in order to eliminate not so that you can still have high boost levels high power and not have this knock destroying your
Engine. Well, you can use you can retard the ignition timing, but that means you're not going to be as efficient
You're not gonna make as much power so you don't want to do that
You can lower the compression ratio, which also means less power overall and lower efficiency
But you will often notice the turbocharged engines use
Significantly lower compression ratios than other style engines and that's to help reduce the likelihood of having knock and that in itself
Makes the engine less efficient
But the real kicker here and the real reason why you may not see great fuel economy
Using your downsize turbocharged engine has to do with the air fuel mixture at these higher loads
So what happens is as you floor it in that small turbocharged engine
It wants to avoid these high temperature scenarios in each cylinder and one of the ways it does that is by using a really rich
Air fuel mixture. So ideally you're running somewhere around fourteen point seven to one air to fuel ratio
That's the ideal amount where you're gonna burn all the oxygen. You're gonna burn all the fuel
It's gonna work out great
but when you're running at high boost levels and high throttle you're going to lower that from fourteen point seven to perhaps
Eleven to one perhaps twelve to one in order to bring the temperature in that cylinder down
And so that might seem strange that injecting more fuel actually brings the temperature down
but it does part of the reason why is as you inject that fuel and it has the phase change from a liquid to a
Gas, it drops the temperature within there
so you're injecting more fuel purely to lower the temperature within that cylinder and as
Of doing this you're able to avoid knock and create lots of power. Of course
The downside is by having that rich air fuel mixture. You're not creating power very efficiently
And so if you're to look at a graph of torque here on the vertical axis and then rpm on the horizontal axis
And you look at the torque curve of a turbocharged engine
So it'll start somewhat low come up as that turbo builds boost and then taper off as it starts to get
You know too much speed for that turbocharger. And so you look at you know, this is what your your peak torque is
What the engine is totally capable of and then if you were to look at you know
How much torque can you create how much torque?
Can you ask for how much throttle can you ask for from that engine before?
It has to start enriching that air fuel mixture in order to avoid knock and it's generally not going to be very much
So that's this line right here
so you can see that you know without applying too much throttle you're going to get past this point where you can still use an
Ideal air fuel ratio and still make efficient power then you're going to go beyond that
And once you're beyond that you're using that rich air fuel mixture, and as a result, you're getting lots of power great
But you're not doing it very efficiently and so with a naturally aspirated engine
Let's say a larger naturally aspirated engine which has its drawbacks. You don't want that many cylinders
You don't want that size necessarily for the low load scenarios. But as you add those cylinders
The amount of work that each cylinder is doing is less the amount of air and fuel going into one cylinder is less
So those cylinder pressures are lower
The temperatures are lower and as a result
you don't have to use as rich of air/fuel mixtures and so by using a
Less rich air fuel mixture in these high torque scenarios in a naturally aspirated engine you're able to get a better
You're able to get more power for the amount of fuel that you're injecting into it
So, you know the benefit of these small turbo engines at low loads you can get good fuel economy
But then once you start asking for those higher amounts of power and order for it to avoid knock
It has to use a really rich mixture and so you get poor fuel economy
Now, why do this? Why go this strategy?
Well from the manufacturers end if they're looking to publish, you know
EPA city and highway numbers and that cycle doesn't have much demand if the demand is all on this low-end
Well, then you can use a small turbo engine to kind of trick that test into showing great fuel economy numbers
whereas if you were to start getting into those higher load scenarios, those fuel economy numbers are going to go down and so
You know
that's kind of an interesting thing where it's a
manufacturer choice of how accurate do they want their EPA published numbers to represent the fuel economy that you're actually going to get and so
The next video that I make and I'll include a link to it
somewhere around here once it's actually published but the next video I make is going to be on
Mazdas
2.5 liter turbo and we're going to be talking about how they actually design in order to raise this level right here at which you
Can have increased loads and still be within that stoichiometric air fuel ratio and still get a good amount of power without
Sacrificing your fuel economy nearly as much as some other styles of small turbos
So look forward to that video if you have any questions or comments, of course, feel free to leave those below. Thanks for watching