series hybrid vehicle

mellowyelloe said:

I'm wanting to build a series hybrid vehicle. Where I'll have gas engine running a generator to power the electric drive motor. In addition I want a small amout of batteries to handle the short bursts ...

Click to expand...

The idea is valid, but what really makes series hybrid worth it, is not a small amount of batteries but a large amount. Those days with expensive and heavy NiMH cells are over. Now it makes economically and technically sense to have 30-50 km of full electric range, and all hybrids should be made plug-in.

This also has the point most people forget to mention: if you buy a larger battery pack, you can get more power out of cheaper low-power cells. For example, if you built a pack that can run 2 km electric, you would need to buy special power cells that are more expensive. If you build a 10x larger pack with the same cells, you get not only 20 km electric range (actually a bit more!) but also 10x higher output power (for higher speeds & acceleration). In practice, if you don't need all that power, you can then switch to a cheaper cell -- from so called "power cells" to so called "energy cell", latter of which is optimized for low weight and low cost (EV use). Larger pack = better battery efficiency, more power (= better acceleration at higher speeds), less need for cooling, and what goes without saying, more range.

Also, the energy recoverable from regenerative braking and the benefit running the ICE at optimum RPM (traditional points of having a hybrid drive with a small battery pack) is not so much; you may lose it in the series hybrid losses. The large battery pack changes the game by making your car a plug-in car. Then you mostly drive with electricity from the wall and only need gasoline for longer trips, and even then only for a part of the trip.

Expect to spend $2000-$3000 in lithium battery cells and you'll get decent electric range and power. You'll get this investment back in about 5 years in saved gas. A proper lithium pack lasts for at least 10 years.

gas engine,the truck would have enough room for the electric motor and a small motorcycle engine(hoping 125-250cc would be ample) to run the generator and hopefully enough batteries for what I'm needing.

Click to expand...

Well, to be really usable, your range extender would need at least 10 kW (13 hp) of electrical power. With your truck, that would probably have you go at maybe 60-70 km/h on average with gasoline. With a largish battery, this would truly be average speed; for example, it would charge the batteries while you stop for a break. (You could also charge from a public charging point at the same time.)

So I guess a 125-250cc motorcycle engine ain't enough, unless it's only for emergencies and you are fine with a "limp mode" or extending the range only a bit. I think you need to go up to around 500cc or more. The engine and the generator will weigh more than 100 kg.

Thanks for any help and/or information, and I hope you all aren't strictly electric.

Click to expand...

Most seem to be, but I'm also thinking about converting a pickup to a plug-in series hybrid with a 10..15 kW generator and full electric range of about 50 km.

mellowyelloe said:

Hi everyone. I'm new here, hoping to get some good info.

I'm wanting to build a series hybrid vehicle. Where I'll have gas engine running a generator to power the electric drive motor. In addition I want a small amout of batteries to handle the short bursts like pulling away from a stop sign into heavy traffic. I'm looking at doing it to my 2000 Dodge Dakota.

Pretty much I'm just wondering if anyone here has done this before and if so how it worked out and any other tips or suggestions people could give me.

It is a truck and I would like to keep the bed open. I don't haul much but I still want to be able to when needed. I also don't like the idea of being limited by range of the batteries. I was figuring that with taking out the gas engine,the truck would have enough room for the electric motor and a small motorcycle engine(hoping 125-250cc would be ample) to run the generator and hopefully enough batteries for what I'm needing.

Thanks for any help and/or information, and I hope you all aren't strictly electric.

Click to expand...

I am not as optimistic as Siwastaja as to the workability of such a conversion. Fuel only vehicles are built with peak power engines because they are the only available powerplant and must be able to provide needed power in all situations. As such they tend to be overpowered.

A series hybrid seeks to replace a peak power engine with an average power engine and using the electric motor and batteries for peak power situations. The average power engine is run at maximum efficiency and is used to provide average power to the electric motor and charging power to the batteries which can provide peak power when needed.

Here's the rub, the replacement average gas engine and generator still needs to be able to generate enough average power to keep the vehicle going. You can get a rough estimate by multiplying the average MPH speed by the average WH/m giving a total amount of Wh needed. So 75 MPH x 1250 Wh/m (big truck going fast) is 93750 Wh. This works out to a 125 HP engine. A quick search for How much HP does a 250cc motorcycle have states that race bike that are highly tuned and rebuilt each week can generate up to 100HP.
BTW you'll need more than 125HP average to cover conversion losses and peak power battery recharging.

The next issue is that a genset than can generate that type of power isn't going to be small nor cheap.

Finally you will need enough battery power for your peak power applications.
Depending on your truck application (towing, hill climbing, stump pulling, etc.) peak power applications could drain your small battery pack and your replacement gas engine/genset may not be able to provide the peak power necessary to get the job done.

I took a quick look at the 2000 Dodge Dakota. The I4 engine is 120 HP. So it is doubtful that a replacement engine and genset is going to be much smaller than that original engine. Take note that a Prius has a 97 HP gas engine and a total HP of 134 (gas+electric). And it's smaller than a truck.

At first blush it may seem that folks are down on hybrids. But the reality is that it's difficult to engineer great savings by trading peak power for an effective smaller average power engine. Batteries really are the most effective way to bridge the gap.

ga2500ev

Yes, you can solve all the problems stated by ga2500ev by having a large enough battery. Then it's primarily an EV, but you don't need to have a very expensive Tesla-sized battery for those occasional longer trips. You MAY need to slow down or have additional charging pauses, but you can solve range anxiety by using series hybrid with smallish engine.

I'd go the route of having undersized ICE+generator and accept the fact that in the rare case I'm going outside the battery range, then I'm going slower and maybe take more pauses. But don't undersize too much. For example, it may look tempting to buy a 2-3 kW genset as they are cheap and small, but it won't do practically any good. Go for about 15 kW and you'll get an usable setup. This would be approx. a 25 hp engine. You may need to DIY because the available gensets are both expensive and heavy.

If you need to regularly take long trips, then forget it and just use an efficient gasoline car. A good option is having two cars, one cheaply converted electric (possibly using an old donor with blown engine that would have gone to junk otherwise) and one efficient gasoline car.

So you would need to start by analyzing your usage patterns and needs. What is your typical daily driving pattern? How often do you need more than 50 km? More than 100 km? More than 200 km? What's your conversion budget? How about your experience, can you build an electric generator?

ga2500ev said:

If one could pull up somewhere, plug in, and get 70 miles worth of energy in 20-30 minutes, then the series hybrid concept would die pretty quickly don't you think?

Click to expand...

Indeed; electric power is everywhere, and it might prove easiest to just install large 3-phase sockets as it just takes a few minutes of electrician's time and costs maybe $100 in parts, compared to a DC fast charge station that may cost $10000 or more.

But then, you would need a quick charger in your car. Power electronics is however becoming cheaper and more lightweight, so having a 100 kW charger with you isn't more expensive or heavy than having a 15 kW genset.

But now it seems that the DC quick charging stations will become more common rather quickly; more quickly than many anticipated. Not where I live, but everywhere else yes!


There is still a difference though. You can make a somewhat usable series hybrid design with a 50 km battery range, but you cannot stop every 50 km for 20-30 minutes to recharge, so in that case you would need at least 150-200 km of electric range. As the battery prices come down, this stops being an issue, and at some point, the genset will be more expensive than just having more batteries and stopping for a quick charge every 200 km or so.

Hi Mellowyelloe

Series hybrids do exist - but only for specialized purposes like diggers, mining trucks and trains

The reason for this is that they are too inefficient
Each time you convert energy the gods of thermodynamics take their tithe

Series Hybrid
Diesel - power (30%)
power - electricity (85%)
electricity - power (85%)
gearbox - road (95%)
Total - 20%

Good old diesel truck
Diesel - power (30%)
gearbox - road (95%)
Total - 28%

You can use a smaller and more efficient (at the lower power rating) engine BUT diesel engines do not have the massive drop off in efficiency at low load you see in petrol engines and the losses from the double conversion are just too great

The series hybrids that do exist are because of the convenience of separating the motors from the transmissions - and having drive on multiple wheels
And are used despite the inefficiencies

Note the Prius and Volt are parallel hybrids as in each case the IC engine physically drive the wheels

Duncan said:

Series Hybrid
Diesel - power (30%)
power - electricity (85%)
electricity - power (85%)
gearbox - road (95%)
Total - 20%

Good old diesel truck
Diesel - power (30%)
gearbox - road (95%)
Total - 28%

Click to expand...

Hi,

Using numbers to prove anything is very dangerous if the numbers are off. Garbage in, garbage out, no matter the formula, but this is especially true when the results accumulate.

First, the ICE efficiency in an electric generator is better than when driving wheels directly as the engine runs on optimum RPM and torque all the time. Second, the electric generator and motor efficiency is better than 85% in a properly designed system. 90% is easy to achieve. (Even 95% for a generator is not unheard of). A difference this small is relevant because it is squared in the equation.

You also forget recovering energy via regenerative braking.

So, fixing the numbers, we get (assuming that your optimum-RPM ICE eff and gearbox eff are correct):

Series Hybrid
Diesel - mechanical (30%)
mechanical - electricity (92%)
electricity - mechanical (88%)
gearbox - road (95%)
Total - 23%

Good old diesel truck
Diesel - mechanical (20%)
gearbox - road (95%)
Total - 19%

This is assuming mixed driving with some city driving where the engine runs on non-optimum rpm and torque (+ idling). In pure city driving the difference is larger. Completely outside the cities, with the ICE running optimally, the series hybrid will probably be less efficient because of the conversion losses you mention, but not overly so. It is important to consider where do you drive most of the time.

The real idea behind a series plug-in hybrid, instead, is to drive from the plug as much as possible, in which case a comparison like this is useless as you don't run the diesel. If you run it only now and then, then it's not very relevant in the big picture.

The only thing I don't like in hybrids is the increased complexity. (And the ICE!)

Okay, so let's get to the basics. To summarize EV vs. hybrid vs. traditional ICE and series vs. parallel hybrid:

EV without the quick charging infrastructure and smallish battery:
Only for city driving

Hybrid:
Mostly for city driving

Traditional ICE:
Mostly for highway driving

Near-future EV with a large battery and quick charging network on its disposal:
Best for everything


Parallel hybrid:
Best when mostly driving with gas
+ less losses when driving with gas (than series)
- more losses when driving with plug-in electricity

Series hybrid:
Best when mostly driving with plug-in electricity
- more losses when driving with gas (than parallel)
+ less losses when driving with plug-in electricity


Production hybrids are parallel because they have evolved from gasoline cars and designed to be mostly run with gasoline, not from plug-in charging. Chevy Volt is a horribly complex design which has both parallel and series features. Quite frankly, I have no interest in debating whether it is a series or parallel hybrid.

From what I have found, gasoline engines range from 20% to 35% efficiency, and diesel engines up to 40% or even more than 50%. And diesel efficiency seem to be fairly flat from idle speed to about 1800 RPM:

http://www.boostmyfuel.com/pdf/efficiency.pdf
and
http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

Have a look at this from http://www.gmc.com/fuel-efficiency/better-diesel-fuel-economy-than-with-gasoline.html:

The MAN S80ME-C7 low speed diesel engine is one of the most fuel efficient engines of its size on the planet. It is a diesel engine that uses only 155 grams of fuel per kWh of energy produced. This gives it an energy-conversion efficiency of 54.4%

Click to expand...

Diesel fuel has energy of about 11.6kWh/kg, or 86g/kWh. Here is a 30 kW diesel genset weighing 390 kg, that has a fuel consumption of 232g/kWh, which is about 37% efficient overall (I presume including conversion to electricity):
http://yxdieselengine.com/product-1-1-generator-set-diesel-engine-en/137780#.UeH3UZ3D_IU

Since most diesel engine vehicles are only about 25-30% efficient, a series plug-in hybrid equipped with a genset as described above would be much better, especially with a fairly large battery pack that could be recharged from the grid for most travel requirements, and the genset could be used for longer range driving where grid power is not available.

However, it seems wasteful to have a separate generator and electric traction motor, so that is where more complex series/parallel systems are probably better. If the diesel engine could be mechanically disconnected from the generator (which would also be a three-phase traction motor), when the batteries are fully charged, you could run just from the batteries. When charging is needed, the diesel engine could be connected to the common shaft (through a variable speed transmission if necessary) to provide enough torque to supply the driving requirement and additional torque to charge the batteries using regen. Since diesels have a fairly flat efficiency curve over a reasonable range of RPM (about 900-1800), it could be engaged for a normal range of driving speeds (30-60 MPH) where it would be in its highest efficiency range and its torque could be dynamically adjusted between traction power and battery charging power.