Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

How a Car Engine Works

jacoboneal.com

Post reply on HN
Print view View original

131–140 of 172 posts

Re: How a Car Engine Works

#131

Post by philsnow »

mercnet wrote: post #117
kirse wrote: post #80

Earlier quoted context omitted.

Yep, Nissan's 370Z has had SynchroRevMatch as part of its Sport/NISMO packages for a few years now, and from what I've heard through friends it's very simple and works really well... Although it makes me wonder at what point on the spectrum does a "manual" become an "automatic" http://en.wikipedia.org/wiki/Synchronized_down_shift_rev-mat...

Sometimes I do not have the right shoes on to rev match so I can see this feature being useful. I would consider a car a manual if the user has to press a clutch in before changing gears.

Right... shoes?

I haven't driven a stick regularly since high school, don't people rev match with their ears ?

Re: How a Car Engine Works

#132

Post by kens »

hnriot wrote: post #118
umsm wrote: post #74

Earlier quoted context omitted.

A professor in college once told me that if you can't explain a technology, you might as well assume it's magic.

that wasn't really very helpful or education of your professor. What, I wonder was, their point? I can't explain why the earth's core is still so damn hot after cooling for several billion years, but I know it's not magic. I could name you many areas that I'm not an expert in, but I know for a fact none of them rely on magic, except possible quantum mechanics. The jury's still out on that being magic.

The cooling of the earth and why the core is hot is actually a very interesting topic. Lord Kelvin (who has the temperature unit named after him) came up with an argument that the earth had to be 20 to 40 million years old if you assumed it started as a molten lump of rock and solved the equations for how long it would take to cool. (This age for the earth made both the evolutionists and creationists unhappy, by the way.)

He had a very clever argument, but the factor he was missing was radioactive decay, which provides an additional heat source. He also carefully figured that the Sun couldn't be more than 20 million years old, but here he was missing the existence of fusion as a power source.

Re: How a Car Engine Works

#133

Post by TomEllis »

philsnow wrote: post #131
mercnet wrote: post #117

Earlier quoted context omitted.

Sometimes I do not have the right shoes on to rev match so I can see this feature being useful. I would consider a car a manual if the user has to press a clutch in before changing gears.

Right... shoes? I haven't driven a stick regularly since high school, don't people rev match with their ears ?

Yes right shoes. If you want to heel toe, you can't do it in your steel capped boots http://www.youtube.com/watch?v=muxVLOaw2ik

Re: How a Car Engine Works

#134

Post by maxerickson »

freehunter wrote: post #116
pdonis wrote: post #114

Earlier quoted context omitted.

Yes, because that means the knock sensor is reducing compression because it senses lower octane gasoline.

Another note I've learned on higher octane gas: in areas prone to freezing temperatures (I live in the northern Midwest), 87 octane will, during winter, have ethanol added into the mix as an anti-freeze. This is commonly called the "winter blend", and is federally regulated to only be on the market during certain times. The ethanol has lower energy density than gasoline, and can hurt your mileage. Higher octane gas d…

It replaced MTBE as an oxygenate. It's present in some blends all the time.

Re: How a Car Engine Works

#136

Post by deepblueq »

grecy wrote: post #94
nexox wrote: post #84

Earlier quoted context omitted.

Actually VW sold a dual clutch DSG transmission in the 2003 R32, years before the GTR hit the market, and for a fraction of the price. All other things the same, a dual clutch is not much faster than a properly-operated manually-clutched gearbox, but in the case of the GTR, we'll never be able to make that comparison.

> a dual clutch is not much faster than a properly-operated manually-clutched gearbox That's not correct. When the GT-R first came out I read a great article about why it was so damn quick down the 1/4 compared to super cars that are 2-6x more expensive. The reasons were: 1. Most of them dyno at around 480hp AT THE WHEELS. So it's making more power than Nissan claims. 2. It's the first car to get all the electronics…

I suspect that a big limitation in shift speed is often the angular mass of the crankshaft and flywheel - that energy has to go somewhere in an upshift, and doing that quickly on a typical car with wide gearing and a heavy flywheel puts an incredible amount of stress on things. Gear ratios cancel out in the real world, since with close ratios you're also shifting more often.

I would expect something like the GT-R to have much less crank/flywheel angular mass than the average car, as well as beefed up transmission components. Even so, IIRC they had to dial things back a bit because transmissions were blowing up.

DCTs get rid of the delay from matching the input and output shafts of a conventional manual. Very helpful, but not a cure-all. My car (automated manual, non-DCT) spends much more time matching the engine to the input shaft than the input to the output shaft.

Re: How a Car Engine Works

#137

Post by L_Rahman »

peterwwillis wrote: post #92
umsm wrote: post #74

Earlier quoted context omitted.

A professor in college once told me that if you can't explain a technology, you might as well assume it's magic.

Isn't that how medicine works?

Yes. As an example, you'd be surprised by how few MDs that are experts in their respective fields have a detailed understanding of the pharmacokinetics of the drug they're prescribing. They just take the dose the drug company did trials at and go with it.

One reason is that it takes a lot of modeling to understand why to dose 1x twice a week vs 2x once a week, so everyone gets roughly the same dose.

Re: How a Car Engine Works

#139

Post by MrFoof »

nexox wrote: post #84
grecy wrote: post #75

Earlier quoted context omitted.

That's one of the major reasons the GT-R is so quick off the line and down the quarter mile. It was the first production car with a dual-clutch transmission that was actually realistically priced, and it's still arguably the most cost-effective.

Actually VW sold a dual clutch DSG transmission in the 2003 R32, years before the GTR hit the market, and for a fraction of the price. All other things the same, a dual clutch is not much faster than a properly-operated manually-clutched gearbox, but in the case of the GTR, we'll never be able to make that comparison.

>All other things the same, a dual clutch is not much faster than a properly-operated manually-clutched gearbox

Formula One drivers in the early 90s were the only ones really able to come close, with shift times between 150 and 200ms. The average person is more around 300 - 400ms when shifting aggressively. You can be faster than 300 - 400ms, but not with 100% consistency, especially on downshifts, and especially with some transmissions with odd clutch engagement points or shift levers that require more deliberate effort.

A modern DCT, like Porsche's 2nd generation PDK, has a shift time of 150ms. Porsche's 3rd generations are even faster -- the PDK-S in the new GT3 is 100ms. The version in the 918 Spyder is 50ms. All of these are perfectly rev-matched, every time.

I've driven many tremendously good manual transmissions. However, generation two PDK is good enough that after four test drives (two with a 6-speed, two with PDK + Sport Chrono) I've actually decided to order my Cayman S with PDK since it's unquestionably better. The only thing I wish is it had the "burnout/donut mode" of the 3rd gen.

Re: How a Car Engine Works

#140

Post by sitharus »

njoubert wrote: post #122
jcampbell1 wrote: post #54

Earlier quoted context omitted.

Direct injection implies the fuel is injected at the top of compression stroke, but the animation shows the fuel being added during the intake stroke. This is just an inaccurate drawing showing the injector attached to the cylinder rather than the intake manifold. Interestingly, this could be a drawing of a ultra modern gas direct injection system running in classic mode. Ford's EcoTech will, under certain conditions…

Beat me to it! This totally amazed me when I started working on cars. Both carburetors and direct injection creates a mixture of gasoline and air BEFORE this mixture enters the cylinder. Direct injection now seems like a misnomer ;) It makes a lot of sense if you think about the incredibly violent conditions inside the cylinder head though. I'm always surprised that spark plugs survive as easily as they do, I imagine…

There are true common-rail direct cylinder injection gasoline engines out there. They're not so common though, since there aren't as many advantages to it compared to a diesel engine.

Cars marketed as GDI, such as the Ford EcoBoost, inject directly in the cylinder.

Post reply on HN