Minimise finished EV weight

Technical discussion on converting internal combustion to electric
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Paul9
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Minimise finished EV weight

Post by Paul9 »

Eng_Capacity_vs_Vehicle_Weight.docx

Hi People,

I have been wondering about suitable vehicles to be used as donor vehicles. I am aware that many factors influence suitability as a donor vehicle but I assume one of the major "criteria" would be due to weight. My logic, which prompted a bit of research, was that, as the ICE engine is probably the heaviest item to be removed from a donor wehicle, the heavier the ICE engine in relation to the weight of the donor vehicle, then the converted ev would be as light as possible. I was unable to find data on engine weights for various donor vehicles but then thought that engine capacity might be the best indicator of engine weight.

My conclusion therefore, and being influenced by a few glasses of red wine, was that the higher the ratio of engine capacity to vehicle weight then the less increase in vehicle weight when the ev components were added.

I researched various vehicles and have attached, as a MS Word doc, my findings ( I found it difficult to "paste" the data into this message as I lost all formatting when I attempted to do so.) A couple of examples inlcuded in the word doc are:

Daihatsu Centro     weight = 630kgs   eng capacity 660cc Ratio capacity to weight = 1.047619
Daewoo 1.5i    weight = 940kgs   eng capacity 1498cc Ratio capacity to weight = 1.593617
Mazda 323 Astina   weight 1014kgs   eng capacity 1840   Ratio capacity to weight = 1.814596

As you can see the ratio of engine capacity (in lieu of engine weight) to vehicle weight varied considerably. The Mazdas had the best ratio and the Daihatsu had one of the worst. I realise that some ICE engines have alloy heads which would vary their weights compared to similar capacity ICE engines without alloy heads.

Is my logic "out of whack" or could this be one way of selecting possible donor vehicles to minimise eventual weight increase in the finished EV?

Thanks
Paul

Tritium_James
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Post by Tritium_James »

Paul,

It's a bit hard to tell just from the engine capacity. The weight difference between an old cast-iron block (with heavy radiator, water pumps, etc) and a modern alloy block (with lightweight bits) is huge.

As a rough rule of thumb, by the time you've taken out all the ICE bits, and then added in the EV bits except for the cells themselves you will be back around the original weight. Then add the mass of the cells to give your final vehicle weight.

So what you're really looking for is a car that can handle your planned weight of cells + passengers + luggage allowance without going over the GVM.

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Post by Paul9 »

Thanks Tritium-James,

So I assume you are saying that the difference in weight of old cast iron ICE engines and new alloy engines is so great that using engine capacity as even a rough indicator of engine weight is almost a waste of time?

Thanks
Paul

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Post by wheresmyEV »

Paul,
I like your idea of trying to calcualte engine weight to try and pick the best donor car. I haven't seen any vehicles of recent years that still use cast iron heads, and I have rarely seen many alloy blocks. I haven't been aware of any recent trend towards alloy blocks, correct me if I'm wrong. There are also other factors to consider which might complexify you calcs slightly but you might also look at accessories like power steering, air conditioning, power windows, etc. This data should be available and you could add or scale some how the weight for these items to try and estimate base rolling weight.
But all this brings another concern to mind. Since the losses we see are going to be frictional (some relationship to mass but not straight forward) and wind resistance. I would see wind resistance as the limiting factor for highway travel. How do we find which vehicle has the best aerodynamis? Perhaps the fuel consumption figures for highway travel and veicle mass might help to determince an equation for estimating wind resistance?
Has anyone tried this?

Regards,
Kent.
Of all the things I've lost I miss my mind the most

Paul9
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Post by Paul9 »

Thanks Kent for your comments.

Your point about wind resistance is obviously valid and there are many factors which influence vehicle "performance". It is hard to judge (I think?) factors such as wind resistance and rolling resistance (tyres) before you purchase the donor vehicle. The factor I was trying to estimate before purchasing the vehicle is weight. There are a number of threads which deal with all these factors and there are spreadsheets which calculate the effect of these factors however I am trying to get an estimate of at least one of these factors before buying the donor vehicle.

Thanks for your input.
Cheers
Paul

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Richo
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Post by Richo »

Aerodynamics will limit your top speed.
Weight will effect your acceleration.
Rolling resistance is small - changing tyres to save 5% of a small value should be a low priority.
It'd be like removing the little "on" light from a room heater.

Ferrari use alloy blocks so best not convert those Image

There is limited info to help you choose a better performing car.
A key would be to look for test drives that say x kW and top speed x km/hr.
This will give a hint to it's aerodynamics.

The weight only becomes an issue if you are expecting unrealistic ranges or using lead batteries.
Just take the leap of faith and buy the car you want to drive and make it work as an eV.

The only real factor to consider is will the car hold enough batteries to have the desired range.
I doubt there is much correlation between engine capacity and engine weight.
Especially when you consider the engine capacity refers to the size of the holes in the block.
It would probably need to be broken down into categories for No of cylinders in-line and V, sohc, dohc.
Too hard.

You would probably have better luck asking a motor reconditioner in another state how much the motor weights for shipping purposes...

Go to a wrecker they probably have stacks of motors laying around.
Just take a set of bathroom scales with you.

I went to wreckers to measure up cars for battery capacity before buying a donor.
Some don't care as long as you don't bug them.

You can only get so much information from sitting in front of a computer
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Post by Paul9 »

Thanks Richo for the comments.

I think weight would also effect range as well as acceleration. I own a property/weekender in the Blue Mountains west of Sydney. The house runs off solar hot water, photovoltaic panels and a wind turbine. Totally off grid. Being impatient to drive something electric while my ev was being built, I bought a 48v DC tricycle with a ute-type tray on the back. It is good for collecting wood for the fire and other jobs around the property. It is recharged by a separate 48v set of photovoltaic panels on the garage roof.

The tray on the back was made of steel which I thought was pretty silly for an electric vehicle. I therefore had a alloy tray built and put it on the tricycle instead of the steel tray. The tricycle weighed 340kgs with the steel tray and now weighs 310kgs with the alloy tray - a decrease in weight of about 9%.

Acceleration improved as did the amp hours/watt hours used to travel each km. I measured the difference in watt hours consumed per km and the improvement was 11% - roughly the same percentage improvement in range as the decrease in weight.

My next "improvement" in weight will be with the batteries. The tricycle is still running on the original 120amphr capacity wet lead acid batteries but they are fairly heavy. When they expire I will replace them with same capacity batteries but lighter. I am hoping for a further 5% to 7% improvement in acceleration and range.

Hence my interest in improving/decreasing the weight of my ev!

Your suggestion about visiting used car wreckers is something I will definitely follow up on!

Thanks again
Paul



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Post by Tritium_James »

Paul, your tricycle is not the same as a road-going EV. Since it travels slowly, there is hardly any aero drag, so nearly all your losses are from rolling resistance. So reducing the mass of the vehicle will reduce the power consumption by a proportional amount. In this kind of vehicle, less weight translates directly to less power consumption and more range.

But in a road car, only a small percentage of the total loss is from rolling resistance (maybe 5-10% at 60-80km/h). Reducing the mass will reduce this part of the loss, but since the vast bulk of the total loss is still from aero drag, reducing weight won't really make a massive difference to the range.

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Post by woody »

I think the most important factor is not weight, nor is it air resistance, it's how much you like the car.

The more you like it, the more you will drive it when it's finished => success.

More km electric vs petrol => more environmental savings.
More km electric => less $/km and hrs/km spent on conversion.
Tritium_James wrote: But in a road car, only a small percentage of the total loss is from rolling resistance (maybe 5-10% at 60-80km/h). Reducing the mass will reduce this part of the loss, but since the vast bulk of the total loss is still from aero drag, reducing weight won't really make a massive difference to the range.
I agree with you a bit James, but I think your numbers are out :-)
(Solar Car figures?)

Maybe my sums are wrong, but Mr Spreadsheet says that that the speed where Rolling Resistance and Air Resistance are equal is between 50 (mini) and 75 (MX-5) km/h.

The speed where rolling is only 10% is 3 times that - between 150kph (mini) and 225 (MX-5).

Even at 110kph, Rolling Resistance is still 20-30%.

Weight also plays a factor in acceleration, braking, cornering, hill climbing.

So I'd say weight is of high importance, but it is hard to get rid of.

Something to not when comparing cars is that the Cd quoted needs to be multiplied the frontal area - so comparing a Mini (Cd 0.53 x Area 1.5 = 0.8) and a Hilux (Cd 0.45 x Area 2.0 = 0.9), the mini gets less wind resistance.

I think in choosing a car, you avoid the ones which are bigger and heavier than they need to be for the purpose (Hummer, 4x4(, and you've saved most of the weight. Older cars tend to be lighter, mostly because they don't have the bits which make newer cars quieter, more comfortable and more safe :-)
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

Paul9
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Post by Paul9 »

Thanks woody and TritiumJames.

I definitely agree with an ev's success being judged by how much you enjoy driving it! I'd enjoy mine if it only got me to the front gate and I had to push it back into the garage!

However I would enjoy it more with more range and performance. It struck me the other day while test driving my 1994 suzuki swift ev that ice vehicles have electric windows. I decided that if I ever win lotto I am going to build an ev with a tiny petrol engine in each door to make the windows go up and down!! :-) Good idea?? Image

Thanks
Paul

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Post by woody »

Great Idea:
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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Post by Paul9 »

What a great ad!!!!

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Post by Richo »

woody wrote: I think the most important factor is not weight, nor is it air resistance, it's how much you like the car.

Agreed - this should be the most important apect when deciding.
woody wrote: Maybe my sums are wrong, but Mr Spreadsheet says that that the speed where Rolling Resistance and Air Resistance are equal is between 50 (mini) and 75 (MX-5) km/h.

Mmmm I Might check this - I have no data with me at the moment.
But common sense would say a 650kg car will have a rolling resistance force of about 90-100N.
So your saying a mini's aerodynamic drag is about 100N @ 50km/h?

With 200N total force @ 50km/hr (13.88m/s) this is about 2.8kW.
or about 3.5kW with motor losses.
So about 70Wh/km?

Plausible
So the short answer is NO but the long answer is YES.
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Post by Tritium_James »

Yes I must have been thinking solarcars, sorry!

The data I have for a Diahatsu Mira shows the drag from rolling resistance is the same as aero drag at 60km/h. Aero is 2x rolling by 85km/h, and 4x by 120km/h.

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Richo
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Post by Richo »

woody wrote: Rolling Resistance and Air Resistance are equal is ~50 (mini)km/h.
Richo wrote: Mmmm I Might check this - I have no data with me at the moment.

Yep looks about right.

But in comparison dropping the rolling resistance by 10N (10%) in a mini sized car you need to loose ~80kg.
And I think most people will struggle to find that 80kg post conversion.

How many batteries is 80kg worth in range?
~6900Wh using TS cells.
Using 150Wh/km this is an extra 40km range.

I think I'd rather have the 80kg/40km of batteries rather than the continuous saving of 10N.
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Post by drgrieve »

Image

Hi I've uploaded a spreadsheet that I've used from diyelectriccar.

It's useful to look at different cars aerodynamics. The kw to maintain speed I think is on the high side - but I don't have any real world figures to compare to.

Enjoy.

Rename the file to .xls - I couldn't see a way to upload a file.
Last edited by drgrieve on Tue, 12 Jul 2011, 17:07, edited 1 time in total.

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