Article: adding a turbo to improve efficiency

[JohnnyGrey]

I would say because you have enlarged the size of the 0.42 island it has the potential to increase FE if you are smart enough to hold the engine in the 2000 +- range that the island falls in

Yeah? What about the other axis you're ignoring? Does anyone here understand what torque is? Anyone?

[diesel_john]

Originally Posted by JohnnyGrey

Yeah? What about the other axis you're ignoring? Does anyone here understand what torque is? Anyone?

i think people are expecting to pulse and glide.

Therefore using the extra torque for a shorter acceleration period and a longer glide period.

more torque of course wouldn't help if you can't use all of it.

With my 71 Lb-ft @3000 diesel my foot isn't on the floor all of the time, therefore the engine is too big, that's my take.

[JohnnyGrey]

more torque of course wouldn't help if you can't use all of it.


with my 52hp diesel my foot isn't on the floor all of the time, therefore the engine is too big, that's my take.

Ding ding ding!!! That's correct.

However, even with a turbo that allows for faster acceleration at the same FC, that value is in Fuel/Power not Fuel/Time, therefore, there is no gain.

In a P&G scenario, with an N/A motor, you might spend 5 seconds pulsing from 55-65mph. In a turbo motor, you might spend 3 seconds doing the same. However, when you reach that 65mph, you will have burned through at least as much (of a more expensive) fuel as the N/A motor. Remember, the map is Fuel/Power, not Fuel/Time! A turbo only makes the sweet spot taller, it doesn't make it sweeter or shorter.

Besides, if you don't know what happens when you suddenly shut off the oil feed to a spooled turbo, let me tell you, it's not pretty or cheap.

[LostCause]

What you are forgetting is that there is more than one way to downsize an engine other than physically.

A turbocharger mated to a late-closing intake cam will see your fuel efficiency increase, guaranteed. The only sticking point is that the turbocharger has to be more efficient at compressing the intake air than the piston during the early part of its stroke.

Pistons are efficient at high compression levels, but superchargers (whether they be turbo or belt-driven) are often more efficient at low compression levels. Mazda used a Lysholm supercharger on their Miller-cycle Millenia engine, but just as efficient of a turbocharger could be found (drivability would go down, though).

The Miller-cycle decreases the effective volume of the engine, making up the capacity difference with a supercharger. Mazda also downsized their engine, but they were interested in total efficiency.

Also, I don't understand why a turbocharger doesn't decrease pumping losses, at least on the intake stroke. If the intake manifold is above ambient, doesn't that mean the absence of pumping losses?

- LostCause

[Duffman]

Originally Posted by LostCause

Also, I don't understand why a turbocharger doesn't decrease pumping losses, at least on the intake stroke. If the intake manifold is above ambient, doesn't that mean the absence of pumping losses?

- LostCause

Because gasoline engines are throttled to limit power when low power is desired. The manifold is still under vacuum up until 100% volumentric efficiency (its probably 85% because most NA engines cant fill thier cylinders 100%) so there is still pumping losses.

I write this under the assumption we are comparing the same engine pre and post turbo.

[JohnnyGrey]

What you are forgetting is that there is more than one way to downsize an engine other than physically.

A turbocharger mated to a late-closing intake cam will see your fuel efficiency increase, guaranteed. The only sticking point is that the turbocharger has to be more efficient at compressing the intake air than the piston during the early part of its stroke.

If you could vary that intake cam on the fly, I could see your point, but why would you want 2.5L worth of metal moving around in a motor that breathes like a 1.8L? So the turbo brings you back up to stock horsepower? Also remember that turbo energy isn't entirely free. That backpressure costs you.

Also, I don't understand why a turbocharger doesn't decrease pumping losses, at least on the intake stroke. If the intake manifold is above ambient, doesn't that mean the absence of pumping losses?

The manifold AT ambient decreases pumping losses and any engine can do this perfectly well without a turbo. It's called flooring the accelerator. The problem, like I've stated many times in this thread, is that I have no way of efficiently using that power. The way a turbo can help, is if I downsize my 2.2 to something like a 1.2T. That way, I can run that 1.2L at atmospheric manifold pressure while cruising, well within the BSFC sweet spot. The only reason that turbo is necessary, is to make the little 1.2L viable for when I need to pass or merge into traffic. During FE cruising, it lies dormant. If I run my 2.2L at atmospheric manifold pressures (with turbo or without), I'll be over 100mph before long.

[JohnnyGrey]

I'll give you an analogy that describes our BSFC dilemma...

Say you need tomatoes, so you go to your grocery store and they have a 2lb package for $6 and a 5lb package for $10.

-2lbs of tomatoes are enough for your dinner
-You could only hope to eat 3lbs of tomatoes before the rest spoil.

What do you do?

The 5lb package is a better deal right? Well if you bought the 5lb package, and you could only eat 3lbs before the other 2lbs spoil, you've effectively paid $3.33/lb for your tomatoes.

[LostCause]

Originally Posted by JohnnyGrey

If you could vary that intake cam on the fly, I could see your point, but why would you want 2.5L worth of metal moving around in a motor that breathes like a 1.8L? So the turbo brings you back up to stock horsepower? Also remember that turbo energy isn't entirely free. That backpressure costs you.

1.) Increased fuel economy
2.) A custom cam is much easier to source then a custom 1.8l engine

Mazda downsized their Millenia engine partially for reduced friction, but they were designing a new engine. My point is that a turbo can increase a full sized engine's fuel economy. Whether it is desirable or not is an individual's choice.

Fuel efficiency had to come from somewhere. A turbo takes energy, but it takes less energy then the piston to accomplish the same job.

Originally Posted by JohnnyGrey

The manifold AT ambient decreases pumping losses and any engine can do this perfectly well without a turbo. It's called flooring the accelerator. The problem, like I've stated many times in this thread, is that I have no way of efficiently using that power. The way a turbo can help, is if I downsize my 2.2 to something like a 1.2T. That way, I can run that 1.2L at atmospheric manifold pressure while cruising, well within the BSFC sweet spot. The only reason that turbo is necessary, is to make the little 1.2L viable for when I need to pass or merge into traffic. During FE cruising, it lies dormant. If I run my 2.2L at atmospheric manifold pressures (with turbo or without), I'll be over 100mph before long.

I understand the function of a downsized-turbocharged engine, but what I don't understand is the variation of pressure with throttle position (assuming constant boost).

Obviously when the turbo isn't producing boost, the engine is essentially N/A, but what about when the turbo is working? I think a turbocharged engine running at part throttle will have an above ambient intake manifold simply because airplanes seem to do it all the time.

A conventional turbocharged engine, which will not require boost at cruise, will not eliminate pumping losses. A Miller-cycle engine, which will require some boost at cruise, should eliminate pumping losses. Therefore, a turbo can eliminate pumping losses...right?

- LostCause

[Chuck.]

Isn't the fuel economy advantage getting a smaller engine than you would have, then add a turbo? In other words, puting a turbo to what you have adds more power - but no fuel economy.

[Duffman]

Lost Cause,
Holding engine displacement, ignition timing, A/F ratio, RPM and compression ratio the same in a gasoline engine the power produced is proportional to the cylinder filling (AKA Volumetric efficiency VE). Because we are holding all the above constant, friction and all the other losses are constant as well and we are really only adding fuel with the increased air (constant A/F Ratio) so power will be proportional. So to put out X hp at say 2000 RPM engine A may require 50% cylinder filling. In reality 100% of the cylinder is full it is just at 1/2 pressure, hence it is operating under vacuum. Most NA engines cant exceed around 85% VE due to restrictive intake ports and cam timing. A turbo can easily exceed 100% VE becuase it is compressing the air (boost) but again really 100% of the cylinder is full but the pressure is above 1 Atmosphere so we can say it has exceeded 100% VE.

Does this help?