Comment by Aurornis | Hacker News Reader

Comment by Aurornis | Hacker News Reader

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You're right that the best Brake Specific Fuel Consumption (BSFC) occurs at higher loads. If anyone wants to see a map of this, search for "brake specific fuel consumption map". More modern engines might actually have their peaks closer to 3000RPM due to improved technology. The trend is toward smaller engines that rev higher, so peak efficiency is steadily shifting to the right.

However, it probably doesn't matter due to the extreme mismatch between the engine's output and the generator's maximum power.

The generator appears to be rated at 5500W, which is 7.4HP. Efficiency of electrical generators is high, so we'll call the load 8HP maximum.

An Toyota Sienna engine has a peak power output around 200HP.

For reference, a car air conditioning system might consume around 4HP, or half of this generator's load. The generator is barely more than running the air conditioning.

So ideally, you'd pick a set of pulleys that let the engine basically idle. The goal is to minimize internal engine losses at this level, because you're nowhere near the peak efficiency islands on the BSFC map.

Wow, I had no idea that a car engine produced more than a hundred kilowatts of power, that's ridiculous! (For comparison, an average household consumes something like tens of kilowatt hours per day, so driving for an hour (I guess at peak, which an engine wont always be) may be similar to 5 or 10 days of a household's power usage, and not to mention much less efficiently generated.

petrol is roughly 9.5kW/litre (~35kWh/US gallon) if that's a better reference for consumption (of which maybe 30% will make it to energy through a modern engine?)

The rediculoussness of the energy use of transport is especially noticeable when you are driving a Tesla which by default shows you a dial of power consumption and generation in kW.

When rolling down a hill with regenerative breaking, the car is generating 50kW.

You can literally power many average homes with that.

On the other hand, pedaling a bicycle, or pushing a car is a lot of work.

It's not ridiculous until you compare against people doing it manually.

also, teslas are really quite efficient and it's amazing how they recover energy as well.

I wonder if there are efficiencies to be had recovering energy to come, since they can only recover what the batteries can absorb.

Pedalling a bike is not a lot of work. Old people do it daily.

Thrusting a car through air at 100kph though, that takes a lot of work. Drag scales with the square of the velocity and proportionally to the frontal surface area.

Cars are a fast, but woefully inefficient means of transport.

> Drag scales with the square of the velocity

Drag force scales with the square of velocity. Drag power (rate of work or rate of energy consumption) scales with the cube of velocity.

Good clarification thanks !

Perhaps you've never cycled up a hill.

Perhaps I've grown up in Europe and have been cycling my whole life.

You could be from Denmark

I'm not, I've just lived there ;)

You lose a lot of energy to drag, friction and resistance(aka: heat.) Battery capacities are almost never the limiting factor in this equation. This is especially so when you don’t top the battery off to begin with.

Of course it's ridiculous to have 300kW car engines racing from light to light averaging 10mph in the city when someone on a bike can do that faster with 100W.

Only if the lights are stopping the car but not the cyclist. If you apply regulations that are designed to favour one contestant, it's not longer a fair comparison.

I walked to work for over a decade before I retired. I stopped at all the red lights and beat the cars most days. Bikes and pedestrians can go much faster than cars between lights when traffic is bad enough. Sure, there is favorable treatment (pedestrians get their own "roads" and bikes can drive between lanes), but there is no similar treatment that would make sense for cars because they are too big and insufficiently nimble.

If anything, bicycles are very rarely (in some places, never) favoured over cars in urban planning/design.

300kw is peak power.

I think teslas are pretty efficient:

https://forum.abetterrouteplanner.com/blogs/entry/22-tesla-m...

A model 3 LR uses less than 100 watt hours to go 1 mile at 25mph

I wonder how efficient bikes become at higher speeds with respect to drag coefficient (without a streamlined fairing)

> On the other hand, pedaling a bicycle, or pushing a car is a lot of work.

One of these is not like the other. Pedalling a bike doesn't come even close.

Humans consume several million calories per day at most, now as a rule of thumb a million calories is about the same as a kWh. And the only reason they consume that much energy is because the human body is inefficient at converting this heat to work. Even the best cyclists struggle to produce anything more than a few hundred watt.

More to the point, humans struggle to power a light bulb, let alone a house.

> And the only reason they consume that much energy is because the human body is inefficient at converting this heat to work.

The human body does not convert heat to work. We do something entirely different (look up the "ATP/ADP cycle").

But yeah there are certainly conversion losses in the conversion of egg sandwiches to labor. 25% or so. Which is actually pretty good; you won't be able to match that with a steam machine built over the weekend (start with solving the problem of setting eggs on fire?).

And in many cases, that much power transports a single person and occasionally a few bags of groceries.

edit: By point of comparison, a good cyclist produces about 100 watts, probably less if they're on a leisurely cruise. That's about 2% of the wattage of a car engine.

For more direct comparison, there are motorcycles on the market that can produce ~150kW , to transport about the same amount of stuff.

Bicycles are really efficient (and those motorcycles are very fast)

> And in many cases, that much power transports a single person and occasionally a few bags of groceries.

Is there any comparable mode of transport that can accommodate all of this in poor weather (i.e. a snowstorm) without having to wear burdensome clothing?

I realize 150 years ago people wouldn’t venture out in those conditions, but they also didn’t move more than 50 km from their birthplace over their lifetime.

Yes: a bus or light rail system can carry tens or hundreds of people at a time for a comparable amount of wattage per person. Build the system underground and inclement weather becomes even less of an issue.

You're still going to want to bundle up for the parts where you're walking to or from the station. But it's not like that's difficult or burdensome to do, at least for anyone hale enough to walk a couple miles without trouble. I commuted to work daily via light rail for years; with a couple of well-thought-out layers, good gloves and shoes, scarf, and hat, you're good to go down to about 15°F in reasonably still weather, and by adding a parka and balaclava you can take that down to 0° and still be quite comfortable - and with a backpack or satchel, carry quite a lot besides.

(If anything, you might find yourself too warm and sweating! Working your muscles puts out a surprising amount of heat, and modern textiles can do an amazing job of keeping it where it does the most good.)

Frankly, this is a skill anyone should have who lives where it gets cold enough to be useful. You won't always get to know ahead of time when you might be out in the cold - after a car wreck, for example, and maybe without a running engine for a heater into the bargain - so it pays to dress for what you could end up needing, or at least have the parts of such an outfit close to hand against the chance you might.

I live in a city that's often said to have the best public transit infrastructure in Europe and one of the best in the world.

I still use my car to go shopping and to transport myself. Carrying bags of groceries from the shop to the station, and from the station to my house is a lot of work and I'm not going to do that in bad weather.

And public transit is much slower than going by car. For example: My doctor (that I have to visit every 2 weeks) is 25 minutes away by car and 1.5h by public transit.

Don't trust stories about how you can work or read in public transit. The vehicles are so full in peak times that you're happy you at least get to fit into the vehicle.

And it's a thoroughly bad experience during summer, I am sensitive to smells and I just can't do it, the body odors are too much and I'd throw up immediately.

I live in a city with one of the oldest but largest transport systems in Europe and if you’re near a rail (under or over ground) stop, public transport is usually quicker. For journeys within the inner zone, cycling is usually the next quickest and often quicker than public transit even. However, once you get to the outskirts (or late at night) and have to depend on the bus network, driving or private hire becomes quicker again.

I'm right next to rail, that's the 1.5h I was talking about. 3 changes on the way and the waiting time adds up.

Reminds me of a city I used to live in. Their bus system was actually really good on average, but the worst-case routes had two 30-minute layovers. Even in -40F drifting snow it was faster to walk across the city than take those trips, and you'd only really take the bus if you had a lot to carry or wanted to avoid the elements.

Where do you life that snowstorms are a daily concern? McMurdo station?

I live in the middle of Europe.

Snowstorms are a daily concern for 2 months of a year.

Extreme heat is a daily concern for 2 months of a year, too.

And rain is a daily concern for 2-3 months of a year.

Nice so you can cycle without worry for about half the year.

Not going to do that with groceries, and generally it takes too much time and effort (this city is built on a lot of hills) and is too dangerous around here. Nicer bikes get stolen a lot (I lost 3 before I lost my patience and didn't buy another one), cars are much harder to steal.

In my part of Canada, weekly or twice weekly snow events are a reality from late November to early April.

Nice so you can cycle for half a year without worrying about snowstorms.

I don’t hibernate for the other half of the year. I continue to cycle when weather permits. I might have been able to carry enough groceries for myself in two panniers, but it’s not practical for enough food my family.

A cargo bike might carry four or so grocery bins, but not the six to eight I now get on my biweekly shop.

The continuation of this thought is to realize what an immense amount of power there is in a gallon of gas. It is basically free energy compared to using serfs or beasts of burden to do the work.

I saw something the other day - the numbers aren't exactly right, but it was along the lines of there's the same amount of energy in a barrel of oil as one man working full time in a manual job FOR SEVEN YEARS.

This is why we've (unfortunately) built modern societies on fossil fuels. The amount of work done as a result of it is immense, and probably explains why there aren't vast numbers of people enslaved for manual labour any more.

It's maybe equally mind-blosing to consider the miniscule amount of mass required to generate equivalent energy in nuclear reactions.

Modern cars are powerful. Today there's no car on the market today that doesn't have enough power. The cheapest cars you can buy, small sub $20k hatchbacks, are all around 100kw. They all have performance figures that beat sports cars from the 1970s.

The limiting factor on early acceleration (0-60kph) today for all cars is now tyres. Beyond that air resistance might make more engine power matter but in general it's all about the tyres. Every car can spin their wheels at the lights if they want to. If you feel the need to floor it at the lights you're showing off your tyres not your engine power (and also wearing them out).

"There's nothing like bad tires to make it sound hot off the line..."

(racetrack joke — — who needs a big engine to squeal the tires when a set of crummy tires will do it even better?)

"There is no such thing as too much horsepower...just not enough traction" ~ Carroll Shelby

Sub $20k vehicles do not exist in this day and age.

[new cars]

Nissan Versa is 16k. Kia Rio is 17k. Mitsubishi Mirage is 17k.

https://www.caranddriver.com/features/g44830210/cheapest-new...

There may not be a LOT of cars in that price range but they definitely exist.

Let me introduce to you tata tiag ev. https://tiagoev.tatamotors.com/

A sub $10k "electric" vehicle that's essentially the best selling electric car in india right now because of it's price.

A 4 star global ncap rated car so not a death box btw

0 to 60 in 5.7 .... a sub 10k car putting up the same numbers as a 1980s Ferrari Testarossa.

I'm embarrassed to have at one point unironically thought of myself as a serious Car Guy. EVs have done so much to expose mainstream male-oriented car culture as an utterly ridiculous and performative costume ball for manchildren.

It's 0 to 60 in km/h* FYI

Oh thanks for pointing that out, I did not even notice they had switched units. When these comparisons are made normally if kilometers are being used as the unit then the zero to 100 time is given to make a comparable to miles. I don't think I've ever seen 0 to 60 times listed in kilometers per hour even in markets where km is the customary unit of distance. Is this something normally done for the Indian market? I suppose as 0 to 37-ish mph time of 5.7 seconds is respectable for travel on surface streets, but my joke comparison to the Testarossa rather falls down now. Assuming acceleration remains linear until 60 miles per hour then this little box will come in at just a hair over 9 seconds. Still, that puts it right in the same ballpark as a V8 powered Camaro from the mid 80s.

this is a family car. it is NOT supposed to be fast, heck it will spend most of its operating life running in less than 60 KMPH.

and yes, 0-100 KMPH is the standard because KM and not miles is the standard.

0-60 is for those chasing numbers, 0-40, 0-60, 0-100, these are all for those who want to advertise how fast a machine is or for breaking records.

indian roads are NOT for driving at 100 KMPH on a regular basis. highways and expressways but that is not normal driving in india

If india is anything like sri lanka, you'd be lucky to make 30 kmph.

Fiat Panda and Renault Twingo start around 50 kW.

If you actually run it at that power constantly it's not going to last long though. It's peak power. You're not going to drag race your car, most of the time it's just cruising.

This is a big difference to (piston) airplane engines because those are designed to give near 100% a lot of the time.

Yeah, marine boat engines (gas outboards -- NOT auxilliary diesels!) are designed to run close to full-throttle ~all the time.

Huh I didn't know that. I guess cooling is simple there considering they can just ingest cool water and expel it without having a radiator. I don't know much about boats.

It sounds easier than on piston planes even. They run pretty hot when running flat-out while parked, like during a run-up test. We were always told to only do it for 30 seconds or so.

They generally use air cooling and despite the huge "fan" on the front it struggles in this scenario. When you're doing 100 knots it's less of a problem.

This is very visible in EVs. They can power a house for multiple days, but last only few hours at highway speeds.

24 kilowatt•hours/day is better known as simply a kilowatt

No.

A kilowatt is the instantaneous consumption of 1,000 watts.

Eh GP is technically correct here if you do the dimensional analysis because the two time units cancel out and you're left with only a power unit.

Saying a household averages a consumption of 1000 watts over the course of a day makes exactly as much sense as saying someone averaged 30mph over the course of a day on a road trip.

Its why having an EV hookup is so funny: your house is now a tiny percentage of your power bill/load dimensioning.

Also why I'd really like to have my car battery be usable by the house: storing solar or winter/night consumption all require a battery. You'll never need a bigger one for your house than the one already in your EV.

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