As I think about it, I’ve never driven a turbocharged car. What I know about turbochargers is this: In an airplane, the turbocharger provides a greater density of air so that the engine can maintain power at a higher altitude. Flying high can get an airplane above weather, and the lower density allows higher ground speeds.
I’ve always thought that in a car the turbocharger provides more power at ground level by allowing more fuel to be burned. That is, if the ideal air/fuel ratio is about 15:1 then adding more air requires adding more fuel to maintain the ratio. More fuel/air mixture makes a bigger boom when it’s ignited, and the energy goes to the crankshaft, right? I think MSNBC is wrong that the air ‘forces the pistons down faster’ (their implication seems to be that fuel doesn’t have anything to do with the extra power), and they’re definitely wrong that turbocharging increases displacement. Displacement is fixed. Can anyone verify that I haven’t had it wrong all this time?
Also: Do turbocharged cars really burn less fuel? As I said, if you increase the amount of air you have to increase the amount of fuel. Do turbocharged cars really get better mileage than normally-aspirated but otherwise identical cars? If so, is it because they get to cruising speed more quickly? (i.e., burning more fuel to accelerate, but burning it for a shorter amount of time than a normally-aspirated car uses to reach the same speed?) Or are they saying that a turbocharged car uses less fuel than a normally-aspirated engine of larger displacement?
Car and Driver had a small article where they compared a 4 cylinder with a turbo vs. a normal V-6 for a similar car. They found that overall the turbo cars got slightly worse mileage. This was based on 3 sets of cars. They were expecting to see the mileages be the same.
Additional power is created by additional fuel being burned. Hard to see how MPG could ever be increased. Turbochargers, superchargers and Nitrous Oxide systems are all used to increase HP. They all do so by (as you wrote) by introducing additional oxygen into the combustion chambers. In turn, this allows more fuel to be introduced with every opening of the intake valve. The optimum air/fuel ratio, BTW, is closer to 14:1.
Interesting note: In WWII the Germans experimented with Nitrous Oxide to increase the high altitude performance of their fighters. The engines were oxygen-starved at higher altitudes which reduced the amount of fuel that could be burnt which reduced power.
If you take a 3.0 liter V6 engine, add a turbo charger: you get more power and your mileage goes down. Forcing air into the engine increases the am’t of fuel that can be burned on each power stroke, so the engine wants more fuel.
Power is increased, and, of course, displacement is not.
Let’s say said 3.0 engine is 250 HP sans turbo (aka ‘‘normally aspirated’’). It gets about 20 MPG in a given vehicle. A turbo, depending on boost amounts, could offer up another 50 H.P. in a modest setup. Fuel economy will probably suffer by 3-4 MPG.
Some will argue that 280 HP from a 3.0 liter V6 is going to be better than 280 from a 4.6 liter V8. It depends. If they are very similar in design, it will be darn close, with the V8 offering more torque and ‘‘driveability’’, but dragging its own heavier carcass around will affect it negatively, and then the vehicle needs a bit more beefiness, too = more weight = lower mileage.
Turbos lag a bit, as exhaust gas, which spin the turbo, needs to speed up before it really offers great boost, and this pushes the power curve up into higher rev ranges, and torque is less present and less useful (higher up in the rev range) than it would be in a larger displacement engine.
But to quickly add power to a given design, save weight and not crush fuel economy, a turbo is nice.
FROM SCRATCH: Better to design a road engine without a turbo. But… Turbos still come from the factory to save weight on high performance cars and daily driver cars to avoid a whole redesign of an engine or model line.
Gasoline engines require that the compression ratio be lowered to avoid detonation, and that negates whatever economy advantages that turbocharging might otherwise yield.
That of course applies to sea level applications. Pilots and Drivers in Mountainous areas may arrive at other conclusions.
Turbocharging is at it’s best when applied to Diesel engines, where detonation is controlled by the rate that fuel is injected. There are probably still a few road and sea going Diesel vehicles (Army Humvees come to mind) that are not turbocharged, but turbocharging has pretty much come to dominate the Diesel market. It allows Diesels to match or even exceed the power density of gasoline engines.
Yeah, that article really is a mess. Youwould do much better to read the article at Howstuffworks.com about turbochargers. Basically they do the same thing on the ground that they do in the air. They increase the air density, thus allowing a greater amount of fuel to be burned inthe cylinder.
For many different reasons, one being exhaust system longevity, many turbo systems now go to “full rich” once the intake system goes from vacuum to pressure (depending on the make of your vehicle, it detects this either via a Mass Airflow, or MAF, sensor, or a Manifold Absolute Pressure, or MAP, sensor). Going full rich will often put more fuel than necessary into the system, and decrease mileage accordingly.
On older turbo systems, this was not done as much. I could easily get better fuel economy on a Chrysler 2.2 Turbo with 175HP or more than similarly equipped cars with V6 engines.
Another thing that has contributed is engine design advances. We now have large displacement V-6 and V-8 engines that, coupled to an aerodynamic body, can get exceptional fuel economy. The 3.5 liter v-6 in my Pontiac G6 gets 30mpg on the freeway with the A/C engaged, and it puts out 220HP. By contrast, the Dodge SRT4 with similar HP ratings doesn’t get much better economy, and it’s a smaller car.
I think what Kevbo is trying to say is that turbo charging a diesel engine will see it behave a bit more like a gasoline engine: It should offer up a bit more horsepower and rev up a little better, rather than dragging through big, long gears as it relies on copious amounts of torque. It should feel livelier with a turbo, and even quieter.
The original article is poorly worded. Turbos do not increase displacement, but they allow the engine to pump more air, which is roughly the same as increased displacement.
The mileage advantage from turbos comes from the fact that a turbo motor can be made smaller, and therefor get better mileage when driving at lower rpm’s, yet produce lots of power when needed. When the rpms are high, and the turbo is producing boost, you are burning lots of gas.
Apropos of almost nothing, once when I was a young man I test drove a Mitsubishi Colt Turbo. It had about 13 horsepower until the turbo spooled up, then had about 193. Accelerating was strange:
A turbocharged engine could theoretically deliver same power/better mileage than a bigger non-turbocharged one because it uses energy that otherwise would be lost as heat (through the exhaust) to run the turbocharger. This is in contrast to a supercharged engine, which takes energy from the mechanical motion of the engine to run the supercharger.
I got my first turbo car about a year and a half ago. I liken the feeling to cruising along on a bicycle, then having someone put their hand on your back and give you a shove forward. The turbo gives your acceleration a noticable boost. It’s not incredibly dramatic, but it’s significant.
Note that there are many different strategies for turbocharging an engine, especially V-platoform engines with dual exhaust manifolds. You can use one large turbo, or two smaller turbos, and the effects will be quite different.
There are many manufacturers now that work to make the turbo undetectable, which means it begins creating boost at relatively low RPM, which means one of the bigger advantages that Gary mentioned, better economy at low RPM, is lost. This mitigates the effects that rocketeer mentioned above, extremely peaky performance, but it does not help your fuel economy any.
Ford’s future plans for fuel-efficient engines (in relative terms) is heavy use of turbocharger style superchargers (turbos are a type of supercharger) combined with direct injection. I think they’re already available on the MKS, the Taurus SHO, and the Edge or Flex, with many more in the pipeline. I’m told that there’s something about them that also reduces turbo lag (haven’t driven one myself).
Now if you’re always spooling up the turbo, you’re not likely to see huge efficiency gains, but then the same thing goes for HEV’s, too.
I had a turbo Audi 5000 back in the 80s. One thing that feels different from a turbo plane, is that you accelerate a lot more frequently in a car. When you accelerate from a stop, the engine feels normally-powered until it hits about 2000 to 2200 RPM, then it’s a kick in the pants!
That article implies that the turbo pressure is what pushes the pistons down harder/faster. That’s funny that someone writing an article could be so clueless.
I would think that a smaller turbo engine, compared to a larger normally-aspirated engine with the same power at cruise, would use about the same amount of fuel. However, the smaller engine, in a car, would spend some of its time idling, such as sitting at traffic lights, and the smaller turbo engine would use less fuel during these times. This is just me speculating BTW.
I have a device called a Dashhawk that plugs into the OBD2 port on my Mazdaspeed3. With it I can monitor the vacuum/boost.
Here is my understanding of it. It reads from -16 or so (vacuum) to +14 or so (boost). When I’m in vacuum (the engine is sucking in the air itself), the fuel economy is much better, I regularly get 27 MPG in normal everyday driving. When you’re in boost, the turbo is blowing extra air into the intake and you get the burst of acceleration, but the car’s computer is also injecting significantly more fuel into the mixture. More power, less efficiency.
You would gain some fuel economy with a turbo four over a V8 but you can do the same thing by bumping down the axle ratio of the V8. A Ford Crown Vic with a V8 will get similar highway mileage comparable to a tubocharged 4 cylinder of similar sized car.
The problem with a Turbocharged 4 is it’s almost always done as a performance option and will geared as such. I’ve owned a Ford Turbo Coupe and it was a blast to drive but it was tough to dog it around town for mileage. The engine would go to boost immediately negating the fuel economy of a 4 cylinder. A turbo diesel would be a different story as it naturally has more torque and it would be easier to keep it out of boost, particularly with an automatic. The best combination would be a turbocharged diesel with a variable pulley automatic so the computer could keep the engine in the most efficient range while using the transmission to move the car smoothly.
A mix of fact and fiction in the quote. Let’s go though the quote one sentence at a time:
Certain kinds of engines provide a speed advantage, as well. “Some engines make cars go faster.” Well, sure. Non-controversial.
Turbocharged engines, for example, force more air into an engine’s chambers than usual in order to push the pistons downward faster. Turbochargers force more air into an engine, yes. But (as nearly everyone has said) the extra air doesn’t “push the pistons downward faster” unless you’re talking about the wildly indirect effect of allowing more fuel, which means more power, which means more acceleration, which means faster pistons. But that’s not what the plain language of the article says, so consider this a clueless goof.
This helps increase displacement while using less fuel. Again, as everyone already said, turbocharging doesn’t increase displacement. It increases power; I suspect the author was using unfamiliar terminology and conflated the two. Turbocharging can theoretically lead to better mileage (see GaryT’s post), and definitely will in some practical circumstances, although certainly not all. However, I again suspect the author was awkwardly wording the thought–correctly related in the next sentence–that a turbocharged engine can be more efficient that another engine of the same power, not another one of the same displacement.
A turbo-boosted V6 engine uses less gas than — but achieves the same power as — a V8. Yes it can, as evidenced by the next paragraph in the article.:
The Ford Flex crossover, for instance, uses a twin-turbo 3.5-liter V6 engine that gets 355 horsepower and 350 ft.-lb. of torque. That configuration improves fuel economy by 10% to 15% over a V8 in the same class–with no sacrifice in speed.
That’s really good gas mileage for that car. It’s 2 generations ahead of my 88 Ford turbo Coupe which was still using a vane meter and batch fire port injection along with a fixed 2 valve cam. You’re driving the same size engine only it has a mass air flow meter, variably twin cam/4 valves, and direct port injection.
I wish they had kept the original style as it was a clean looking sleeper. Now it looks more Subaru-ish.
You’re not going to see that efficiency because the 2 smaller turbo’s are going to go to boost quickly. It’s tough to tame a turbocharger because it uses a mechanically sprung waste gate and you can’t get one that will dump at zero boost. You can fool a waste gate into runing higher boost by bleeding off the boost signal but you can’t fool it into a lower setting and you need enough spring pressure for it to seat under low RPM operation. It’s tough to keep an engine like that out of boost.
It’s not that it can’t be done but people don’t buy 365 hp cars to see how to squeeze a nickle out of the gas tank. The Ford SHO was built to play and it was a way of getting big block V8 performance under the hood of a Taurus.