Car engines - some tech questions...

[Phil_Stein]

Hopefully there are some gearheads lurking about...

OK, so it seems to be pretty well understood that, for a given car, if you want a more powerful engine (standard gas engine), it will be more expensive and less fuel efficient. Your big V-8 will not, IIUC, be very efficient even if you're not at the drag strip, but rather, driving identically to how you would have driven with a 4 cylinder engine.

I don't know much about how engines work, so explain this to me simply.

The higher expense of a more powerful engine is because, I assume, more powerful engines are larger (more metal), and perhaps more complex (more cylinders).

Less fuel efficient isn't quite as intuitive. Given a 200hp engine and a 100hp engine, both in the same car model, to tool around town at 30 mph, both will be well under their maximum power. So I'm guessing that a big 200hp engine at, say, 20% of max power consumes more gas than a 100hp engine at 40% of max power? If you understand this stuff, please confirm, and, if possible explain a bit as to why this is so.

Also, coming from a technology focus, where most things get better at a ridiculous rate - often doubling along key dimensions in a matter of a year or two, I wonder what the state of technological progress is in internal combustion engines. Are these things getting better? How much more fuel efficient is a typical car engine of a given size, weight and power output compared to a similar one from 10, 20, or 30 years ago?

I know "power" is a bit of a crude term - there are, I think, multiple ways to define it - again, any help here would be nice.

Part of this is brought on by watching the Barrett-Jackson auction recently. Various muscle cars from the 60s and early 70s were sold. Horsepower ratings sounded pretty high, and, IIUC, these cars had performance that is at least respectable compared to modern cars. In the tech world, most tech from the 60s, or even 90s, would be largely a nostalgia or curiousity thing nowadays.

Basically, I guess, I'm looking at car engine tech from a computer guy's standpoint and saying, why doesn't it get a lot better, a lot faster?

[noun]

Because there's a limit to how much 19th century technology can be improved?

[Brandon Clements]

Part of this is brought on by watching the Barrett-Jackson auction recently. Various muscle cars from the 60s and early 70s were sold. Horsepower ratings sounded pretty high, and, IIUC, these cars had performance that is at least respectable compared to modern cars. In the tech world, most tech from the 60s, or even 90s, would be largely a nostalgia or curiousity thing nowadays.
Performance, yes, to some extent. Fuel efficency, emissions, and safety standards? Good luck with that if they're stock. That's the answer to that part of question (and the early 70's oil embargo pretty much killed the muscle car of that era).

[Fugitive]

From a high-level view, your rate of fuel consumption limits the amount of energy available to convert into motive force, and the mass of the car and that amount of energy will limit the acceleration. Even at 100% efficiency there would be a hard limit to what you could do, and they've already been working at a high percentage of efficiency for quite a while. If you're practically capped at 60% efficiency due to thermodynamic losses, and you're currently at 50% efficiency, nothing you can do is really going to make a dramatic difference.

You can scale things up to larger amounts of energy, but driving a nitro funny car to work might affect your gas bill a bit...

Edit: Things seemed so dramatic when it comes to computers simply because we were working at, say, 0.00001% theoretical max efficiency, and we get all excited when we double our CPU speeds, but now you're still only at 0.00002% efficiency.

[Phil_Stein]

Yeah, I can see how there'd be a top-end limit on fuel efficiency at the engine level for a conventional gas engine, though I'd be interested if anybody had data on how close we are and how much we've improved in recent decades.

I'm also interested in the other stuff I was asking above - basically, if you want an engine with higher top-end power in your car, why do you have to sacrifice mid-level gas efficiency to get there? I would assume that engine weight plays some role - an engine with more top end power will be heavier, and thus drag down vehicle efficiency a bit. But I suspect that's only a small part of it, and would be interested in a good explanation of the overall trade-offs.

[Fugitive]

It's not really my area of expertise, but you made me curious, so this is what I could find:
At slow speed (i.e. low power output) the efficiency is much lower than average, due to a larger percentage of the available heat being absorbed by the metal parts of the engine, instead of being used to perform useful work. Gasoline engines also suffer efficiency losses at low speeds from the high turbulence and head loss when the incoming air must fight its way around the nearly-closed throttle

In the case of sports cars, it's exacerbated by being tuned to have a narrow power band. The power band depends on a lot of factors in the engine design like the valve timings and exhaust venting. A sports car is tuned to be optimal within a tighter RPM range, and if you're operating outside that, there'll be energy losses due to the engine not quite performing the cycle optimally (e.g., maybe the exhaust valve opens a ten thousandth of a second later than optimal and a bit of energy is lost compressing the exhaust instead of pushing it out). That's where you see improvements in fuel economy from things like variable valve timings.

Gearing would also play a role since it would pull you in and out of the optimal power band as your speed changes. A lot of the fuel economy charts have a 'ringing' effect that's probably largely due to that.

[nKoan]

Yeah, I can see how there'd be a top-end limit on fuel efficiency at the engine level for a conventional gas engine, though I'd be interested if anybody had data on how close we are and how much we've improved in recent decades.

I'm also interested in the other stuff I was asking above - basically, if you want an engine with higher top-end power in your car, why do you have to sacrifice mid-level gas efficiency to get there? I would assume that engine weight plays some role - an engine with more top end power will be heavier, and thus drag down vehicle efficiency a bit. But I suspect that's only a small part of it, and would be interested in a good explanation of the overall trade-offs.

Actually engine weight isn't everything. An LS1 engine from a recent Corvette is big displacement, big horsepower motor, but since its made from aluminum, it is actually lighter than steel block 6 cylinder and even a lot of 4 cylinder engines.

So, the efficiency question? Power comes from displacement (ignoring turbos and superchargers). Bigger motors like the said Corvette motor has 6 liters of displacement. Your standard 4 cylinder econobox will have 1.8-2.4 liters. In a normal V8 engine all 8 cylinders have to be fed fuel and more gas is used compared to the 4 cylinder engine, if we are talking low/moderate speeds.

Now, displacement doesn't have a direct correlation to fuel consumption, but its a good rough comparator without getting into other things like compression ratio and drivetrain power loss and other factors. Some manufacturers are better at tuning their fuel efficiency for the specific application. Sometimes a 4 cylinder 2.4 liter engine can have better mileage than a 1.8 from a different manufacturer (or even the same manufacturer given a different application). But, all in all, more displacement generally means more power and more fuel consumption (excluding other power adders like super and turbo chargers).

Cadillac with their Northstar engines in the early 90s, experimented with a technology that would shut off fuel and air to half the cylinders to help increase efficiency under low/moderate loads. A lot of other companies have followed suit since. For example, if you have a massive 5 liter engine, the engine will act more like a 2.5 liter engine at low/moderate load. The pistons in the deactivated cylinders are still moving (so there is some wasted energy versus a regular 2.5L 4 cylinder) but you will save a bunch of gas over a different 5 L engine without the technology.

This is much like how the Prius doesn't even use the gas motor until it reaches 30 mph or whatever. In fact, I think the Accord hybrid does this too, giving you effectively pure electric, electric + 1.5L 3-cylinders or electric + 3.0L 6 cylinders, depending on the power you need.

Also, engines have gotten more efficient in the past few years, but weights have gone up dramatically due to safety regulations. Most cars these days are over 3000lbs. In the 80's we were probably talking around 2100lbs. The extra weight means you need more horsepower to travel at equivalent speeds, which in turn means worse mileage. For the most part, engine technology has done a good job keeping up, but there haven't been any huge leaps in a while.

Now, to talk about turbo chargers and super chargers and fuel efficiency and power. In the sense of fuel economy, the turbo/super chargers act a lot like the displacement on demand stuff. At low/moderate loads a 2.0 L turbo charged engine sips much like a non-turbo charged engine of the same displacement does. When higher loads are present, then it starts to drink like a 4.0L non-turbo engine. The benefit is that fuel mileage is comparable to how much you put your foot down.

For example, I have a 2.5 L turbocharged engine. When I'm driving to work, I'll get on average just over 20 mpg with mostly city driving (if I keep my foot out of it). Cruising on the freeway without a lot of stop and go, I can get upwards of 28 mpg. On open track days, where I'm running closer to full throttle for most of the tank at 80-100 mph top speeds, I'll get closer to 12mpg.

As for rating numbers, especially horsepower numbers, those can be quite deceiving. The old muscle cars of the 60s and 70s, while they had big HP numbers, the rest of the car were pretty crummy. But that's why they were popular, big hp numbers in relatively inexpensive cars. The tradeoff was that the other parts were usually from the generic parts bin, and not necessarily well designed for the muscle car. The suspension design and frame construction from that era wasn't that spectacular. Many of these old muscle cars can't take corners at decent speeds, and have to slow down much more when entering a corner. A modern 200 hp Mazda Miata can probably get around a lot of road courses faster than a lot of the muscle cars from the 60s. But, of course, the modern muscle cars have had the same improvements over time, and you could say the same thing about a 06 Mustang GT versus a 66 Mustang GT. And in my mind, since I come from a road racing background, lap times are a much better indicator of overall performance (all other things, like weather conditions and driver, being equal).

Furthermore, with modern technology, horsepower numbers have been increased to ridiculous rates compared to the 60s, but not in consumer cars. In the racing world, top fuel dragsters have 5000 hp and above, but only for 3-5 seconds, and they don't burn the same gas you can buy at the pumps. Also, the tolerances are razor thin and the engines essentially need to be torn down and inspected and rebuilt between every race.

But, in terms of passenger vehicles, the hp numbers haven't changed much, but performance is increasing in other areas. Fuel efficiency is getting better too, but the cars are getting heavier, so its not as noticeable. Also, every generation of a car needs to be bigger and have more power for marketing reasons. A lot of the times, though, even though a subsequent generation is bigger, heavier and more powerful, it tends to get roughly the same miles per gallon, due to the increase in fuel efficiency.

Um, I've written a lot it looks like. Heh, but I can keep going if you have more questions.

[Pogo]

The efficiency and power output of an engine is such a complex matter that it's really not possible to directly answer the question. I think nKoan did a great job breaking things down between the differences and the affects that different configurations may have.

What I didn't see mentioned was timing, crankshaft weight, and fuel delivery, which play a huge role in determining the efficiency and power of an engine. All of these things are made according to the application for the vehicle.

The increased weight of the components in a larger displacement engine make the ultimate efficiency of the engine lower than a lower displacement engine. You have to deliver more fuel into a larger displacement engine in order to achieve the same horsepower as the smaller engine. You lose more power on the intake stroke as you have to pull in more air and fuel into the chamber to achieve the necessary explosion to override a crankshaft which has 5,7,9, or 11 other pistons and bearings on it.

As you no doubt know by now, your question is very complex :)

Combustion engine principles haven't changed much. Neither have the heads or engine block designs. What has mostly changed in the past (and nKoan will probably agree here) is:
a)quality of the machined metals
b)fuel delivery systems which are now computer controlled and highly adjustable in real-time
c)computer controlled timing/ignition systems
d)air flow systems including header and muffler designs and implementation
e)highly complex simulation of combustion engine principles helping to narrow down a "sweet spot" in efficiency between all components

Essentially, the 4 cylinder Honda Civic is (or was for the longest time) the ultimate consumer-car combination of all those principles.

[nKoan]

Absolutely. When I was speaking of increased fuel efficiency that's exactly what I meant. I was staying away from the specifics of what affects fuel efficiency because I had already written too much :p Tires and wheels play a big part in fuel efficiency too.

Also, rotating pieces are more important to efficiency than static weight. A crankshaft with a few ounces shaved off it will have a greater overall effect than taking pounds off the engine block itself.

[Phil_Stein]

Tell me more about vehicle weights.

nKoan - you attribute it to safety regs. Is a car at a given size (say, a small sedan with wheelbase of X) really substantially heavier than a similarly sized sedan from 30 years ago? Or is it just folks upsizing their vehicles - driving SUVs and such?

[nKoan]

I don't mean to say it in a negative light, by the way, because most of the safety improvements I think are very good for passenger vehicles. And yes, its not all safety equipment (airbags and side impact fortifications) but also creature comforts like leather, 6-CD Changer, DVD players, GPS navigation, heated seats, etc.

Also, its a complicated question, because tastes in cars have gotten larger in America in 30 years. For example, the first generation Camry -- 1982 -- was 2300lbs. The current one is 3280lbs. The difference in length is 175 inches versus 189.2 inches. (stats from Wikipedia (http://en.wikipedia.org/wiki/Toyota_Camry)). The first generation Ford Taurus was 3,050 lbs in 1986, and is now 3,741 (FWD version). Again, overall length increased from 188 inches to 202 inches (Wikipedia (http://en.wikipedia.org/wiki/Ford_Taurus)). Look at the 1986 Ford Taurus. 188 inches long and 3050 lbs versus the current Camry (debuted in 2006), 189 inches long and 3280 lbs. That's one inch longer, but over 200 lbs heavier.

Like I was saying before, cars are always a little bit bigger and a little bit more powerful with each generation, so its expected that the size will creep up over 20-30 years. So, the weight gain is partially from the increases in overall size of the vehicles. But, its also because cars are more loaded with gadgets both for safety and for comfort.

Also, some safety improvements actually do reduce weight. Aluminum panels can be stronger, crumple more effectively around the driver and actually be lighter than their steel counterparts from years past. But its also more expensive, so not as widely used.

[Phil_Stein]

Well some of it, I think, is model drift.

IIRC, Accord started off as a smallish car from Honda. It got bigger over the years, and then there was the Civic as the smaller car from Honda. That got bigger, and I think there is some other smaller car from Honda now - don't quite remember though.

i.e. Even though a given model name may now be bigger than the same model name 30 years ago, that doesn't necessarily mean it's a strictly comparable car within the manufacturer's lineup. That said, yeah, the average new car/light truck sold now is probably longer than a few decades back.

[nKoan]

Absolutely its model drift within model lines. But if you take similar overall length cars (like say Old Taurus/New Camry comparison above) you'll see the newer cars are generally heavier than their older counterparts.

[FoRmaT]

Also, new cars come with hundreds of meters of wires and cables. They are more reinforced, what with side impact protection bars etc. Even the car's seats are more and more high tech. Just lift a seat from a '83 VW Golf and then lift one from a '09 model, the new one will probably weight three times as much. All those airbags and electrical motors also add a lot of weigth.

I find it crazy to see how nowadays a BMW series 3 is so expensive and high class that BMW needed to introduce a series one, which is probably also quite a bit more expensive than previous 3s, while today a 3 costs the same as a 5 used to cost.

A few years back, I was playing with the idea of buying a new Alfa 164, with a 1.6 liter engine with 120 hp. The car had a leather steering wheel, red-stitched seats, aluminium wheels... in general, it had a lot of nice and sporty features. It cost 2,1 million pesetas, which is €12,6 k. I bet that for this money, today I'd be looking at a 75hp Fiat.

[SpoofyChop]

Is this research for Muscle Car Tycoon?

:)