ME Magazine Gasoline vs. Diesel Engine Article

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ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Wed Sep 12, 2012 8:48 pm

This is an interesting article talking about the two different types of engines and the pros & cons of each and why we haven't seen significant changes/improvements in fuel economy/efficiency.

http://www.main.experiencetherave.com/t ... rticle.pdf
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by mike-tracy » Thu Sep 13, 2012 12:12 am

That is an interesting article. I still want my ee20 turbo diesel boxer, though. :)
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by jamal » Thu Sep 13, 2012 6:56 pm

I feel like he really glossed over modern engine technology. Every manufacturer is using some combination of turbo, DI, and variable cam timing. And 7:1 CR for a turbo engine?

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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Thu Sep 13, 2012 9:11 pm

The actual research may be dated, because I agree some of the more modern technology isn't discussed. However, none of that technology is going to drastically change the internal combustion engine, they are just incramental improvements.

Yes....when I too read 7:1, that didn't sound right.
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ME Magazine Gasoline vs. Diesel Engine Article

Post by DerFahrer » Fri Sep 14, 2012 12:25 am

Yeah, this article is ignoring a bunch if new technology that's actually already been in service in passenger vehicles for years.

I honestly believe direct injection is more than incremental.

For example, the Hyundai Sonata/Kia Optima Turbo is running a 2.0L DI turbo engine that makes 274hp...

... on 87 octane.

That's as much as an Evo makes, or my twin-turbo B4, and they both have to run 93 octane, and get worse gas mileage.

Ford's EcoBoost V6s are running some 13-14lbs boost, I think, on 10:1 compression. They're 3.5L DI V6s making as high as 365hp, and they'll run on 87 octane too.

Also, Nissan and BMW have variable-valve timing that's so advanced now, that they only have throttle bodies for safety, in case the VVT system fails. You're essentially controlling the valve timing with your right foot.

I don't see why a combination of those two technologies couldn't yield a spark gasoline engine that's as fuel-efficient as a diesel.

Something else that bugs me about that article: Internal-combustion engines are heat engines. The greater the temperature difference between the intake air and the exhaust air, the more efficient the engine is. Actually, if internal-combustion engines could run as hot as possible without damaging themselves in the process, they'd be at maximum efficiency. How has this guy concluded that diesel engines are more efficient because they run cooler?


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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by evolutionmovement » Fri Sep 14, 2012 2:13 am

I think he means there's less energy wasted as heat.

What I think is greatly overstated is pumping losses and engine efficiency. The important thing is that a vehicle is an entire system, not just an engine, and the thing to strive for should be the overall efficiency in how it results in mpg, not a % number on a page. Not to say an increase in IC efficiency isn't important to the end result of resource usage, but it doesn't always translate as such. Engines can only be downsized so much before the required load on them under normal conditions causes the fuel use to exceed the supposed benefits of the greater efficiency. That's why there seems to be a brick wall of fuel efficiency for non-hybrid gas cars of similar shape and weight (as dictated by regulation and marketing concerns) where a smaller engine doesn't get better mileage or minimally more than a larger one that can pull a taller gear. Power/RPM of the engine also has to be factored against the weight and drag of the vehicle. Running a smaller engine at a higher load reduces pumping losses, but it's not necessarily enough to make up for the greater fuel use due to the higher rpm. Who cares about a couple % efficiency when it screams away, can barely accelerate the car, and still gets worse or only marginally better mileage than a more sensibly sized engine. I have my reservations about Ford's EcoBoost 3 in a car such as the Fusion, or even the Focus—the 160Hp 2.0 is slow enough—though the maybe the turbo 3 will actually have some bottom end and widen the torque curve enough to have a meaningful impact. Cars aren't aircraft that run at a fairly constant load and throttle opening. Sure, a car can be built and a small engine matched that can supply the given required HP (plus a small margin for headwinds, inclines, etc.) to cleave through the air a given speed (*cough EPA test), but if the power curve sucks so much that it has to run higher rpm (more cycles=more fuel/unit of time and time at a given speed=distance and that all=lower mpg regardless of a couple, essentially meaningless % gain in efficiency over a larger engine running under less a load and at a lower rpm due to its ability to pull a taller gear) more of the time (WOT more often, lower gearing) or it goes out of its power curve when people drive at differing speeds to the ideal used in testing, the mileage will suffer. It's how Corvettes can manage 40 mpg in good conditions that my lighter probably comparable cda Focus would maybe get 43 or so. Running the allowable tall gearing and under little strain for its enormous engine, a Corvette's engine would barely be turning over, incurring its greater pumping losses, but breathing less air also means burning less fuel (in a modern engine) while the Focus is running, say, 66% higher rpm from the necessarily shorter gearing and at a higher load (more fuel burned per cycle) to make the same amount of power. Of course, the Corvette can use a LOT more fuel a LOT quicker if driven in such a manner, but it also delivers the performance. I'm not advocating everything have an LS V8, merely using two common polar opposites to illustrate my point that BSFC efficiency does not necessarily equate to fuel efficiency.

I don't see why throttles can't be ditched with modern computer control and I'll bet that's the next big thing after DI is more distributed. There's really not a lot left to increase efficiency of the IC engine at this point, beyond the far more expensive direct valve actuation. Unfortunately, relatively poor efficiency or not, the IC utilizes a common fuel that has a shit ton of potential energy so even poor use of it results in more potential work than any alternative yet presented. It's not going anywhere soon. IMO, the best place to look for gains is in the rest of the vehicle. Reduce aero and weight demands on the engine and that downsizing isn't so painful, not painful at all—give the 2.0L the lazy job the Corvette's V8 has. This is what I'm working on.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Fri Sep 14, 2012 3:06 am

Michael,

DI and VVT may be a little more than incremental, and the combination of the technologies may produce the most fuel efficient and powerful engines to date, but I still think there is room to grow....and there may not be one massive change, but several smaller changes/improvements in technology that get us to that next level. The bottom line though is....out of the different types of "engines" the internal combustion engine (ICE) is quite inefficient in comparison to say gas turbines. For ICE’s most of the energy is turned into heat. I don't know if we'll get to the point where we'll have a "major" single break through. Maybe a "combined cycle" setup for the ICE or another power source will replace it as the front runner for the automobile. Unfortunately or fortunately....depending on how you look at it, I think those scenarios are probably beyond our lifetime. But who knows...

Regarding your comment on temperature, I don't necessarily agree. I broke out some books I have on thermo and engine cycles and remembered that it’s very difficult to simplify the efficiency. You essentially have two factors working, you have the efficiency of the engine cycle (Otto cycle for spark ignited engines) and the Diesel cycle for diesel engines. Then you have the efficiency associated with the combustion of the fuel, and then any device that may capture waste heat (i.e. a turbocharger).

This is the blurb from the article:
Diesel engines, on the other hand, tend to run lean, meaning there is more oxygen in the mix than fuel. This reduces in-cylinder average temperatures. Why? The cylinder holds extra air (not just oxygen) per unit of fuel. This lowers the temperature per unit volume of air. Since the gases in the cylinder expand nearly twice as much as those in a spark engine, this results in significantly cooler exhaust temperatures. Moreover, nearly all diesels recover some exhaust to power their turbochargers, further enhancing their efficiency.
The long and short of it, I interpret this as there is less energy being wasted as heat and as a result of that, more energy is being put into doing useful work, hence efficiencies will be greater.

We could spend days upon days discussing this, and there’s a lot of material that’s over my head, or haven’t touched in years.

This entry in Wikipedia is pretty good for further reading.

http://en.wikipedia.org/wiki/Four-stroke_engine

edit: Looks like Steve posted while I was typing this up....
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Fri Sep 14, 2012 3:15 am

Steve,

Great points! I agree that more attention needs to be paid to overall vehicle efficiency, and things like that unfortunately require more in-your-face changes that involve the end user, marketing, styling, design, etc. The engine is for all intent and purposes is a black box under the hood to most car owners. So what the manufacture does to it really doesn't matter as long as the driver can press the go pedal and the car moves.
Josh

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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by DerFahrer » Fri Sep 14, 2012 5:26 am

Yeah, that makes sense, Josh. I understand what you're saying.

I also agree with Steve that there is a threshold of 'over-competence', if you will, where the effort to make an engine as efficient as scientifically possible isn't necessarily worthwhile in a real-world application.

However, all that said, I'm not quite ready to retract my statement, though, for two reasons:

- I'm aware what I'm about to say is an ideal, impossible situation, but if a diesel engine were allowed to operate at the highest temperature it could, again ignoring engine longevity/reliability, the fuel would combust more completely and cause even more expansion of the air, which would do even more work with still the same amount of fuel initially injected into the cylinder. Thus, I'm still convinced that the hotter the engine, the more efficient it is.

- A tiny bit of the hybrid 'regeneration' idea is possible with a non-hybrid ICE. I've been semi-habitually downshifting when I've been slowing down to stop (in the press cars I've been driving, not my B4) instead of using the brakes, as I'm aware that slowing the car down with the brakes is taking the kinetic energy of the moving car and transferring it into a system that I know for a fact will waste 100% of it. So, by letting the drivetrain slow the car down, which generates friction heat in the drivetrain, I'm keeping some of that energy in the system it originated from and can use to power the car. Of course, I'm aware that the car is designed to maintain a certain temperature of all the drivetrain components and will likely vent most of that energy anyway, and also that doing that may be putting extra wear and tear on the drivetrain. But over an entire tank of fuel, I do notice usually a 1 mpg increase (maybe even 2 mpg, but that's very rare) when I do that.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by evolutionmovement » Fri Sep 14, 2012 1:07 pm

In a completely unscientific study I conducted, I found coasting in gear whenever possible gained several mpg over being on the gas more like a normal idiot and coasting in neutral down hills or to stops gained maybe twice as much as in-gear coasting averaged over the course of a tank. With the DI Focus as the test vehicle you're looking at ~30-32 idiot mode, 33-35 in-gear coasting, 34-37 neutral coasting. This was compared over several tanks to each and daily using the neutral coast has given me the consistent 34-37 over 40k miles, but the routes, weather, and traffic vary considerably. That said, I'm pretty confident the neutral coasting is most advantageous.

As for turbines, they are more efficient, sure, but even if they were attached to a CVT to better keep them at their efficiency rpm, they still use more fuel/unit of time/distance travelled. The problem is that the power requirements of the vehicle are far less than that provided by the turbine most of the time, so it sits burning fuel for power it can't use. The efficiency advantage in terms of % or potential energy converted to work is great, but not useable in this case. An automobile's fuel efficiency is down to using less fuel, however that is achieved, not so much how much of that fuel's potential energy is used. The 4-cycle by the real world concerns of unit fuel use/mile (not efficiency of energy conversion) and its easy throttleability is why it rules. Since it only fires each cylinder once every other revolution, it actually doesn't consume much fuel, unlike more power efficient turbines, Wankels, and 2-strokes and their more constant draw of fuel. Even with aircraft, turbines are big fuel guzzlers compared to 4-stoke IC, but that extra power can be used to go faster and/or carry greater payload, the trade off of which can make them more efficient in that application.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Fri Sep 14, 2012 5:38 pm

Regarding turbines, you could however use them to power a generator, which then supplies power to an electric motor. In order to keep the turbine loaded it could divert unused power to batteries to keep them charged and/or a load cell of some kind. This way, your transients are being absorbed and the turbine is running at a relatively fixed & steady state load.

I know I've overly simplified the explanation of the system, and that this new system adds several layers of complexity over a standard IC engine....but it just goes to show why we haven't seen any huge changes in the ICE.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by DerFahrer » Wed Sep 19, 2012 3:36 am

Plus, that's more chances for other inefficiencies to leech some of that energy. I'm just guessing here, but after that surplus energy goes into the battery, and then into the electric motor, and then into the wheels, I would imagine it wouldn't be much more efficient than an ICE hybrid.

Steve, I'm not refuting your numbers, but that just makes zero sense that neutral coasting would use less gas than in-gear coasting. That seems to make it pretty clear that the injectors don't shut off when you're coasting in gear, which is kinda dumb in an MT car, if you ask me, unless there's something about DI that makes that impractical. I know that GM is doing that in their manual Cruze Ecos for sure (which aren't DI); I remember reading a press release about it.

Tell you what... it's really hard to get this kind of information out of an automaker for obvious reasons, but if I run into a knowledgeable Ford engineer that I can get to talk, I'll ask them about that.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by Legacy777 » Wed Sep 19, 2012 6:20 pm

I seem to recall reading something a while back about some guys that got really good gas mileage, but the things they did weren't exactly always safe or recommended.....and one of those things I had thought they did was coasting in neutral.

Most cars have deceleration fuel cut that will cut out fuel when coasting, however you're still turning the main shaft on the trans at the engine speed. So you will have more friction losses at 3,000 rpm than at idle speed.
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Re: ME Magazine Gasoline vs. Diesel Engine Article

Post by evolutionmovement » Wed Sep 19, 2012 9:02 pm

I've done the unscientific testing and that's what I've found and it's something I was surprised at myself, but I'm also not saying it's absolutely certain with my tests being highly varied tank-to-tank. But, what I think is happening is more that I can coast in neutral much farther than I can keeping it in gear before having to be back on the gas again because the high compression ratio gives too much engine braking. So, maybe idle uses so little gas vs. acceleration that the difference in distance traveled is where the benefit comes in.
Midnight in a Perfect World on Amazon or order anywhere. The first book in a quartet chronicling the rise of a man from angry criminal to philanthropist. Midnight... is a distopic noirish novel featuring 'Duchess', a modified 1990 Subaru Legacy wagon.

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