I have heard this discussed before in the not so distant past, so not a new idea here or anything..


We all know we need to get smaller engines, smaller displacement and less weight in order to punch in a couple more MPGs right..? But no one wants to deal with the fact that with smaller engines, it's just downright miserable. Hearing the damn thing struggle with all its might just to move you forward at least the same speed as the traffic around you sucks.

Turbos... Replacement for displacement? General consensus: Semi-Acceptable at best, not an exact replacement.

But damnit if they don't do a pretty damn good job. They're very efficient devices, using just waste gasses that your car doesn't need anyways and turning that energy (literally) back into some more power.
This gives the driver the ability to use or "activate" power when he/she wants to. Want more speed? Put the pedal down harder, spool up the turbo and almost instant power increase. Lay off the throttle and stay out of boost and you have yourself your old 1.6l or 1.8l engine gas savings. It's really pretty simple.


I would assume the only downside to this is the gas octane needed. I don't know if they have any basic low-boost setups which wouldn't need anything higher than 87 octane gasoline.


What do you guys think?


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Just FYI, I get 17mpg when i'm heavy boosting runs through the city. When highway cruising I CAN see 30mpg, although I can't stand driving that consistently boring. Max my car has officially seen is ~28.5MPG(give or take a .1) but i had a few 70-120mph runs in the tank, 2 other people in the car, half full trunk, and tolls and pit stops. If i eliminated the women, the junk, and didn't get so antsy on the foot, i could guarantee 30mpg.

 

i dunno i think the problem is that fuel only has so much energy per unit in it. X amount of fuel when burnt perfectly produces Y amount of HP. Doesnt matter if you have a 6.0L V8 or a turbo 2.5L, 500 HP requires a crapload of fuel.

Same on the smaller scale... doesnt matter if its a 900cc single cylinder with a Turbo, or a 1.8L, 110 HP requires about the same amount of fuel, its the efficiencies in how its burnt, and how the energy produced is transmitted to the ground that is important.

With more efficient burning process and a more efficient method of getting the energy from the combustion to the wheels you could use less fuel to produce the same effect.

 

You're looking at it the other way.


I'm saying OK. We need to get a car that makes 170HP because that's the acceptable norm right now and we feel this is enough zip to get us by (assuming the car's weight is kept rather low.)

We can take a 1.8l engine and slap a turbo on it and by golly, we have ourselves the 170HP. the 1.8 by itself would just make an "ok" 135 HP. Both engines are exactly the same, except one has a smart ECU that activates the boost when throttle and rpms get high enough by closing up the wastegate. Instant 35HP increase in power, whereas to get the same effect we would need to get ourselves something like a... 2.5l engine.

The 1.8l normal would get around 35mpg.
1.8l T would get around 34mpg, as long as it doesn't get into boost which shouldn't even be that hard anyways.

2.5l wouldn't even be able to get anything over 30, no matter if it's in the same exact car under the same exact driving conditions and same driver.




And yeah, on the other things i didn't argue against, I can see your point but I think due to these inefficiencies and capabilities of engines, the turbos "unlock" this extra power by benefiting from the waste gasses. Exhaust is just waste turned into energy. Kinda better than nothing.

 

This right here is the flaw in your logic... and while you're right in that it IS the "acceptable norm right now", it's still flawed logic.

I'm driving a car that gets 106 hp and 103 ft/lbs of torque from 1.5 liters. When I want it to, it will propel me to 60 mph in under 9 seconds... which might not seem fast by today's standards, but it's plenty fast enough to keep up with (or ahead of in most cases) traffic (if you are so inclined), and if you study the statistics from the muscle car era... it's as fast or faster than all but the fastest classic muscle cars of the "golden era" of the late 60's and early 70's! 10-14 seconds to 60 was pretty typical for the average family car back then... but, 9 seconds is "not fast enough" for people today?

When driven without concern for economy, this car will get 28-32 mpg. That's right, it will keep up with traffic, it's nimble and fun to drive, and it will get you over 30 mpg without even trying.

But, if you add a little bit of maturity and drive with a mind toward economy, you can still get where you're going and get around 40 mpg!

You can't do that with 170 hp. 170 hp is not what we "need" in this era of $4/gallon gasoline.

If Americans weren't so anti-diesel as a whole (and who can blame us after what we had to put up with in the 70's and 80's?), the answer is small-displacement turbo diesels. Using my car as an example again... same car with a 1.4 liter turbo-diesel instead of the 1.5 liter gas engine... a little less HP (but we all know that TORQUE is what really matters) at about 85-90, but gobs more torque at 120+. Net result, a more drivable car! And it can get over 60 mpg! I'd have bought one in a heartbeat if they were available here.

 

Wrong on every point.

Engine vacuum is the enemy: The more vacuum created, the less efficient the motor is. One reason is because the dynamic compression ratio is low, another is because vacuum requires energy to create. The most efficient parts of the motor are in the higher load points. How many 18 wheelers do you see cruising in high vacuum? Semi's and the like make the motor just barely large enough to cruise, and use a turbo when acceleration is needed. In a car with a smaller engine, turbo or not, will be more efficient since the motor will spend most of its life in a more efficient powerband (less vacuum).

Smaller engines have less internal friction: One of the sources of friction in the engine is piston rings / wall contact. Basically we want the maximum amount of piston surface area, with the least bit of circumference, since the circumference is the friction surface. The area / radius (a/r) ratio increases linearly. An 80mm piston as an a/r of ~20mm2, and a 160mm piston has an a/r of ~ 40mm2. The larger piston will be more fuel efficient because it has less friction. Larger pistons also require lower pressures to make the same amount of torque. So inherently with no other changes, a single cylinder engine is more fuel efficient than a comparable twin. Smaller engines also tend to have smaller main and rod journals (less friction).

Also worth mentioning, when in boost, a motor is generally less efficient than a naturally aspirated motor at the same power level, because the motor is usually run richer in OEM applications (except with some of the new gas direct port injection stuff). This is usually offset by the fact that 99% of the motors life is spent in vacuum. On the flip side, with forced induction, you are increasing the power output, with almost no increase in internal friction. So it varies from motor to motor. However still saying 500hp requires x amount of fuel regardless on the engine is ignorant to say the least. You're pretty much ignoring every aspect of engine dynamics at that point.

 

what happend in the 70's that made people hate diesel's? I love diesels.. Its my dads work van with a 6.8L v8 towing a 5k pound trailer every day and really not aiming for fuel economy averages 15mpg.. best I saw was 18.. Thats fucking amazing if you ask me

 

Diesel cars in the 70's, the US models in particular, were poorly thought out. Even the decent ones (from Germany and Japan) were smelly, noisy beasts that people just didn't like having to deal with. Probably the worst thing for the diesel rep came with the GM diesel 350. They took a gasoline engine (the good ol' 350) and converted it to diesel... the results were unimpressive.

So, anyone who lived through the 70's and 80's is likely to still harbor some bad feelings toward diesel cars.

 

Not so much, way to over engineer and over think.

A 6.0L V8 running about 500 HP and a Turbo 2.5L running about 500 HP utilize similar sized injectors. Why?.. because 500HP in either platform requires similar amounts of fuel. Yes what you were saying is correct, diff engine sizes have different efficiencies and yep, Turbo engines are usually more efficient.

However, the energy content of Gasoline is relatively standard (assuming we use the same Gasoline in both cars). So the amount of energy released in joules, or kw hours is about the same...it's how the engine gets it to the pavement that differs, and that is where your efficiencies come into play.

At a macro level though both engines require large injectors, not the exact same size because at the micro level the efficiencies are different, so are the tunes and a thousand other small things, but the end result is making large amounts of HP requires large amounts of gasoline any way you want to dress it up. Adding a turbo will not suddenly make a 500HP car get gas mileage like a 125 HP car, UNLESS you are only using 125HP. Either way that has nothing to do with the Turbo and more to do with how the driver drives the car.

You made some great points though and were pretty much spot on, just going in much deeper than needed in my opinion.

 

"Similar" is generic and open to interpretation If by similar, you mean within 20% then i will agree with you.

However the bulk of my argument is that the smaller turbo engine will get better gas mileage at the same cruising hp because the motor is more efficient.

 

and yeah your correct, it is more efficient. if you need 45 HP to keep you cruising at 60 MPH then it will use less gas to do that.

However if your making 500HP, the difference is not a lot... your both gonna use a shit ton of fuel.. lol