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Old 03-22-2005, 01:35 PM   #1
curtis73
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Hybrids from an engineering standpoint

I want to open a dialogue with you all about Hybrids. I personally feel that they don't work and waste energy. I'm gonna post my thoughts and I want some unbiased rebuttal. Please, nobody saying that their Hybrid is the pinnacle of whatever just because they recently shelled out the cash for it. I'm talking objectively. Remember as you type... keep it engineering-based, so I'm talking hypotheticals, not "honda is better than toyota," or "but the Prius is so quiet."

Here's why I don't think they work as far as saving energy. It is a chemical and physical proof that anytime you change states of energy, you lose some. Most of the time its lost to heat, either directly (like a wire getting hot) or indirectly (an electric motor's bearings getting hot from friction). It gets lost other ways, too, but let's look at it just as "energy," not necessarily as heat, kinetic, electric, chemical... just energy.

In a regular vehicle, the energy changes states once. The energy in the fuel is converted to motion. Throughout the rest of the driveline, some of the motion is converted away through friction and heat, so the original energy package gets diluted, but not converted as part of the function of operation.

In a hybrid vehicle, there are two state changes. The energy from the fuel is converted to electricity, then the electricity converted to motion. Each time it changes, it loses energy.

Let's take some hypothetical numbers. Let's say you have a hybrid vehicle with a 50 hp engine. Until you convert that to electricity, let's say you're down to 40hp worth. Now you have to convert it to motion, so maybe only 30 hp makes it to the ground. Small engine, good MPGs, mission accomplished, right?.

Why not make a 50hp car that's not a hybrid? Then the customer is happy since they're still getting the good mileage of a 50 hp engine, but now they're getting 40 hp to the ground. Or, why not make a 40 hp car non-hybrid? Then they still get the same performance at 30 hp with better mileage than the hybrid.

I guess what I'm saying is, why on earth would it be more economical to change states of energy twice? Why not just do it once? My theory is that it was just a marketing ploy, but if you folks know something I don't, spit it out. Engines like the VW TDI have proven that we can get 80 hp to the ground at 45 or so MPGs, so why are we even bothering with expensive, complicated hybrids that get 50 mpgs?
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Old 03-22-2005, 01:39 PM   #2
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Re: Hybrids from an engineering standpoint

Good observation Curtis!... I worked on one of the early hybrid car projects at the University I was attending back in the late 60's. There are actually two other key points that do make the hybrid car viable:

1. Because you change the way the vehicle is powered(electric motor), you can now take advantage of regenerative braking which is "free" considering it was being thrown away as heat previously.

2. You can run the internal combustion engine only when you need to and only where it's most efficient. If the IC Engine was the prime mover, you don't get that choice because you give the operator a throttle pedal and he/she decides. If you have a hybrid, the computer decides how the engine runs-- it is most efficient at wide open throttle and under higher loads.
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Old 03-22-2005, 03:17 PM   #3
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Re: Hybrids from an engineering standpoint

Firstly, sorry its been so long on a post, and I do have a picture of the Fiero curtis, just haven't had time to get it posted up. That motor did detonate internally. You suggested a northstar, but I'm going ahead and doing a Supercharged 3800 swap, Ill keep you posted on that.

In second with craigs statements, I think of it like this. Where as a normal car loses energy every time you decelerate, a hybrid is able to aquire this energy and store it in electricity to it. Yes, while there is always loss of energy, electric motors have now become efficient enough to imploy into such a way.

One other thing, you state an electric motor making 50 hp, then being reduced to 40hp. Where are you getting this reduction? An electric motor doesn't have to undergo through a driveline like normal IC motors, usually there is an electric motor on every wheel if not at least two on a vehicle. I know the newer motors on the drawing boards in michigan are at around the 50ft/lbs of torque mark. They are geared at the wheel, but I don't know if they are seeing such a loss (for a 20% loss like that resembles the loss of an inefficient automatic or CVT)

My two cents
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Old 03-22-2005, 05:30 PM   #4
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Re: Hybrids from an engineering standpoint

So, with the regenerative braking and the fact that the electric motors are direct drive... with those benefits, you're saying that a 50 hp engine in a hybrid car would get is more energy efficient than a 50 hp engine in a non-hybrid car? I know there are way too many factors to say definitively, but in a controlled experiment, same engine, same car... yadda yadda.

I guess what I'm thinking is this: Of the 100% of the energy stored in the fuel, a percentage makes it to the ground creating force, and the rest gets peed away into the world via heat, noise, etc.

Of the BTUs available, do hybrids really get a greater percentage to the ground than the old fashioned engine, tranny, and gears? My intuition says no, but I'll admit that the manufacturers have been rather tight-lipped about the finer points of the technology, so my research has sorta stalled.
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Old 03-22-2005, 05:50 PM   #5
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Re: Re: Hybrids from an engineering standpoint

Quote:
Originally Posted by CraigFL
1. Because you change the way the vehicle is powered(electric motor), you can now take advantage of regenerative braking which is "free" considering it was being thrown away as heat previously.
Quite true and it is completely wasted energy with plain old brakes. But, energy out can only be a function of energy in. Of course you start with fully charged batteries, but you can only make as much electricity as fuel you burn. You can recapture some of the energy you used to make you go when you stop, but the returns are much smaller. I see the benefits to all of the conservation techniques the hybrid uses, but in my little head I can't see how the extra energy changes can be offset by regenerative braking. I'm sure its possible, I just remember chemistry and all of the energy that gets lost in something like that.

Quote:
2. You can run the internal combustion engine only when you need to and only where it's most efficient. If the IC Engine was the prime mover, you don't get that choice because you give the operator a throttle pedal and he/she decides. If you have a hybrid, the computer decides how the engine runs-- it is most efficient at wide open throttle and under higher loads.
I can definitely see the benefit of that... the computer can be programmed to make those decisions. I can safely assume that most of us aren't autistic savants who can read into a real-time 4D fuel map and decide when to do it on our own

Ok, here's another conundrum that comes to mind. You take a Prius and a Jetta TDI in Phoenix, AZ and drive them both to Flagstaff, AZ. Its about a 7000-ft climb in about 140 miles. After the trip, which one will get the better mileage, and will the Prius be able to recharge faster than it needs the power to maintain highway speed? And... if the Prius lags behind, is this an indicator of the hybrid's long term feasibility, or just a kink that needs to be worked out?

I used this comparison since they have similar outputs, similar weights, and other things in common, except the diesel is the current paradigm of simplicity while the hybrid is expectedly complex.

It makes a crystal clear choice for me if I were buying in this segment, which makes me think that hybrids are in part a marketing ploy. Just a thought.
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Old 03-22-2005, 09:14 PM   #6
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Re: Hybrids from an engineering standpoint

Quote:
Originally Posted by curtis73

In a hybrid vehicle, there are two state changes. The energy from the fuel is converted to electricity, then the electricity converted to motion. Each time it changes, it loses energy.
I agree with all the comments in this thread. However, I think the above comment does not apply to the Honda systems. To the best of my knowledge, by far the principle source for electricity in the Honda hybrid is regenerative braking, and such times when the engine has excess kinetic energy which is being wasted, such as when slowing down or backing off the revs. All other times, the engine does not spin the generator, because, as curtis suggests, the engines power is much more efficinetly used in actually moving the car down the road.

I would agree that the notion of using a gas engine to spin a generator becuase the gas engine is too feeble on its own (Prius) is inefficient.

These hybrids usually get better fuel mileage in the city than on the highway, because a substantial amount of the braking energy is reclaimed, in the city, but the aerodynamic losses from highway driving cannot be reclaimed.

I think this is an interesting indicator of how inefficient city driving is for the traditional car.

Also curtis quite correctly discusses the energy loss in using kinetic energy (the spinning crankshaft) to create electricity.
To the best of my knowledge, this is called induction loss. About 75% of the kinetic energy is lost due to induction, that is, a hybrid can only reclaim about 25% of the braking energy, store it and deliver it through an electric motor.

Now, there are other ways of storing kinetic energy, over the short term, that are more efficient. GM has considered using compressed gas and fluid to store and re-deliver the energy. Their systems and more efficient than a hybrid and can be adapted to older vehicles and to existing designs. They tend to be a bit bulky so are most useful on trucks.

Finally, different layouts of hybrid technology can provide other benefits than just better economy.
I believe there is a hybrid of the Lexus RX 300 suv (Car&Driver) where the electric motors/generators work on the rear wheels only, combined with a standard FWD gas engine. In this case the electric motors provide a full 4wd capability and faster acceleration (when charged) that a standard RX330, without actually consuming more fuel.
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Old 03-23-2005, 08:05 AM   #7
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One of the things I dislike about hybrids over diesels is the battery factor. I know batteries have come a long way, but they will still have to be replaced, and it will cost a decent amount. At what point is battery replacement going to cost more than the car itself? I guess that's more financial/environmental(throw-away car) than engineering, but it's the engineering that creates the issue.

Another (albeit not a big deal) thing that bugs me is the hybrids generally have uber-skinny tires and are relatively light cars to begin with when compared to say a VW TDI. I think if you could take one car, and try both options(diesel and hybrid), you would have two cars that drive very differently, but have about the same performance(in terms of power and mileage) with perhaps a slight advantage to the diesel when it comes to passing power.

Then you have cars like the Accord hybrid- it can shut down certain cylinders when cruising to save fuel, and the engineers used new weight-saving techniques in the car to make up for the batteries... so why not do that for ALL the cars, instead of just the hybrid. You have a car that is significantly more complicated than a normal Accord, gets maybe 25% better mileage, and a lot of that improvement is from non-hybrid engineering.


Ok, moving on... this one is hard for me to put into words, so bare with me.

A car has a certain kinetic energy when moving, which is either lost through heat when you brake, or in a hybrid is captured by the regenerative braking. But to me, that sounds like you have to drive inefficiently to make the hybrid work at all! If you want to drive efficiently, you drive so that you avoid braking completely.

Taking off from a green light, with another stop light one block away. In a hybrid, you accelerate and then have to hit the brakes and come to a stop at the next light to save/transform some of the kinetic energy the car had.

In a non-hybrid, you just accelerate more gently, or perhaps at the same rate but to a lower speed. So the next light turns green before you get there. Ideally you don't have to brake at all, so you maintain all of your kinetic energy.

If you drive a hybrid that way, you're not using the "hybrid" at all, you're just driving a car with a tiny engine.

To me, a hybrid sounds like a crutch for wasteful driving, not a wasteful powerplant.
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Old 03-23-2005, 08:08 AM   #8
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In our computer model of a hybrid car, it was best suited for urban driving(no long term freeway jaunts). This partly was because of the dual energy conversion losses that Curtis mentioned. Ideal sizing of the engine was based on the maximum HP required to sustain top speed driving(this is where you need the HP) including the losses. Energy storage devices were then used to improve the driveability as well as recover some of the energy lost in slowing down. Ideally, the engine would run only at wide open throttle and shut down during times when the extra power wasn't needed.

Following the "you have to spend money to make money" idea, you do have to expend energy to make energy. While it may look inefficient at times, overall it should be better.

Another thing you have to consider is that we are at the very beginning with the practical development of hybrid cars. Technology has advanced quite far since I worked on concepts almost 40 years ago but nothing speeds up the development like actual working vehicles. As these vehicles develop more advancements will occur to overcome problems. Selling vehicles like this are part of a long term development process.
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Old 03-24-2005, 09:52 AM   #9
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Re: Hybrids from an engineering standpoint

Gasoline-hybrid cars usually show a decreased fuel consumption similar to that one can get with a diesel engine.

A hybrid works best when driven in the city with accelerations/braking and standstills where the engine can be shut down.

A generator and a set of electric motors are a quite efficient transmission system, both works with high efficiecies which gives a high total efficiency. I would assume that the total efficiency of the system will be close of that we see in a mechanical transmission system. The highest efficiencies could probably be reached with one electric motor on each wheel hub, this would also allow a chassis control system to control torque at all driven wheels.

When a battery is charged and then discharged some energy is lost, but the benefits of the better use of the engine together with the energy recovered during braking will be much larger than the total losses.

As known, an otto engine is most efficient when working at high load, then we see efficiencies of over 30% while the same engine at part load may be just 15% efficient. This means that the mean efficiency of an engine used in a car is quite low, and the more powerful engine you have in relation what is needed the lower the mean efficiency will get. When driving at highway speed a typical car won't consume much more than 10-20hp, so fitting that car with a 200 hp engine is bad from an efficiency standpoint.

Today, with regards to cost, waste material from the car and emissions I would go for a diesel with a particle filter instead of a gasoline-hybrid, the exception being larger cites where the traffic can be slow or stand still for periods of time. The latter is also what for example Toyota Prius was designed for.

The hybrid technology can also be applied to compression ignition engines (the benefit is however smaller due to the better part load efficiency of the CI engine). If one builds a hybrid vehicle with a methanol fueled CI engine one will have a car that can beat the fuel cell cars in terms of efficiency for decades to come.

When it comes to store energy for short periods of time there are also lighter and more efficient options than batteries. In space flywheels made of carbonfibre are sometimes used. There have also been experiments of using a flywheel for energy storage in vehicles.
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Old 03-24-2005, 12:40 PM   #10
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Re: Hybrids from an engineering standpoint

Yeah, I was reading about some of the mass transit busses in Europe being driven by a small (like 40hp) diesel engine that spins a huge flywheel. The engine would store energy in the flywheel at stops for use in between stops.
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Old 03-25-2005, 07:56 AM   #11
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Re: Hybrids from an engineering standpoint

As we all know, invent the most efficient hybrid car and we will soon find something else(non hybrid) that gets better fuel mileage. It is this iterative process and the competitive challenge that creates advancements. Look at all the recent innovations in magnet and battery technology that are the result of competition. We are nowhere near the routine production of hybrid cars. There are many theories as to how they should be done and continuing advancements that will change those theories. With the increasing price of fuel, I would suspect that we will see more and more innovations over the next few years.
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Old 03-25-2005, 01:27 PM   #12
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Re: Hybrids from an engineering standpoint

I think the railroads have done a lot of the research on this for us. They switched over to diesel electric years ago. This allows them to design the diesel/generator system to run within an optimal throttle range and use direct drive electric motors for the motive power. If this system could be modified for automotive use remains to be seen. Cars go through a lot more throttle changes than a train. But it seems to make a lot more sense than what the car manufactures are offering now. Just my
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Old 03-25-2005, 03:33 PM   #13
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Well...we first need to look at how we get our power VIA battery or engine to the ground and what devices we use to create this motion.

in normals cars that use a mechanical drive train we normally assume 12-15% loss manual/automatic or 15-25% AWD.

So what are the losses involved with using electric motors to movie the wheels and how the motors go about putting their motoin to the wheels.
Do we start at the back of the engine where the motor powers the transmission to the drive train or is their a motor at each wheel? how much energy is lost transmiting the power though cables...and how efficient are the motors at turning this energy into motion?

my questoin is if you have a 1200 HP engine would it be better to use the standard drive train or use a DC generator at the back of this engine to transmit power over cables to the motors to power the wheels..or can you get 1200 HP to the ground with DC motors are their even 1200 HP motors??? <---sorry off track.
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Old 03-26-2005, 01:30 AM   #14
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Re: Hybrids from an engineering standpoint

This is really good stuff. I might have to add this link to the sticky at the top
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Old 03-28-2005, 04:15 PM   #15
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DC locomotives are generally 3600 hp max with 6 axles(or motors), 4 axles having found too slippery. Modern DC locos have engine connected to AC alternator, then rectify the AC back to DC. There are also AC locos (6 axle) with upper limit of 6000 hp, but generally most customers take 4000 hp units. AC locos are becoming more popular despite increased first costs because of better traction, power handling, and reduced maintenance. It has been popular in Europe to have mechanical coupling to drive wheels.
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