What is everyone's current thoughts on the "Best Small Car"?

[ik9000]

diesel works by compression driven auto-ignition. I.e. compressing to the point where the fuel spontaneously combusts. Hence no spark plugs like a petrol engine.

This reaction works better in a hot engine, where the heat affects the combustion reaction and the pressure required. So a diesel engine that has been running for a while burns more completely, responds quicker and the exhaust gases are hot enough to burn off residue in the catalytic converters and particulate filters on the way out. If all you do is the school run and popping to the shops it may well be a better idea to not get a diesel.*

* it may also be a good idea to not use the car for the school run at all and simply walk instead. Or if they're old enough, send them on the bus.

[kalniel]

Petrol cars are actually pretty efficient at flat our acceleration. Diesel cars are really good at low load steady state, eg medium speed cruising on the motorway. Hence for start/stop town driving (like I do) diesels really aren't very good.

Fundamentally, in a system that uses stored chemical energy to move it, acceleration needs you to burn a load of that chemical whatever it is.

In my case I think it's mainly torque related - the diesel cars I've driven were traction limited quite early in the rev range and maximum acceleration (ie booting it) occurred generally only over about 2000rpm in total (about 2k to 4k rpm) Taller gearing meant this covered quite a range of speed and at 4k you'd change gear again since torque dropped off, so actually the different in engine speeds between booting it and not wasn't all that different and didn't affect economy much.

On the other hand petrol car booting it means revving quite high - 3.5k to 6+k rpm in my usual cases, and it's this that kills the economy compared to a normal drive of 1.5-3k rpm.

[Zak33]

If all you do is the school run and popping to the shops it may well be a better idea to not get a diesel.*

120% agree

with particulate filters clogging up on nearly every town driven Diesel especially autos and those driven gently by older/cautious drivers, Diesels are hell

If gran or cautious parent driver asks to buy a diesel for the school run, any half sane sales person suggests a petrol. Because otherwise the punter is back within 6 months with the lights on the dash and the car is limp mode, needing a re gen... or someone to drive it hard and fast until the engine management allows it to pick up engine speed and then burn off the crap in the filter.

Me?

Never had the issue. But my cars get driven hot and hard daily.

[kalniel]

If gran or cautious parent driver asks to buy a diesel for the school run, any half sane sales person suggests a petrol. Because otherwise the punter is back within 6 months with the lights on the dash and the car is limp mode, needing a re gen... or someone to drive it hard and fast until the engine management allows it to pick up engine speed and then burn off the crap in the filter.

So the other half of the sales people encourage it for the repair work coming their way

And then there's NOx...

[abaxas]

So the other half of the sales people encourage it for the repair work coming their way

And then there's NOx...

Just as petrols have a massive particulate problem........

Evil gas or cancerous particles. Hard choice eh.

[kalniel]

Just as petrols have a massive particulate problem........

But they don't, that's diesels again.

But point was diesel town driving is also particularly (haha) bad for NOx production. Something my councillor is ignorant of in her quest for 20mph limits.

[Zak33]

So the other half of the sales people encourage it for the repair work coming their way

And then there's NOx...

no the other half of sales people tell people what they want to hear.. sell more cars to people but never get repeat business.

sales people don't get repair work.

[abaxas]

But they don't, that's diesels again.

But point was diesel town driving is also particularly (haha) bad for NOx production. Something my councillor is ignorant of in her quest for 20mph limits.

ACtually not, most modern direct injection petrol cars produce more particulate matter than modern diesels do. They have no particulate filtering.

[DanceswithUnix]

ACtually not, most modern direct injection petrol cars produce more particulate matter than modern diesels do. They have no particulate filtering.

If the Euro 5 particulate cap was the same for both diesel and DI petrol back in 2009, then that seems an odd sweeping or just out of date statement?

https://en.wikipedia.org/wiki/Europe...tandards#Euro5

Specially when a petrol engine with no attempt to control particulates would AIUI meet diesel Euro 3 regulations, which seems reasonable as those were the regulations in place for the first DI petrol engines I have come across.

A quick Google did turn up an article saying that petrol cars should have DPF filters, but that said that adding the DPF filter would only cost 50 Euro. Knowing people who have written old diesel cars off rather than replace the DPF which couldn't be unclogged, either that is a really small DPF filter as it has so little work to do, or the writer is clueless.

Am I missing something?

[kalniel]

ACtually not, most modern direct injection petrol cars produce more particulate matter than modern diesels do. They have no particulate filtering.

That's because they don't need it. Only more recent DI engines produce any noticeable particulates, hence the limit coming in, but they're easily way lower than diesel and don't need filters to meet the limits.

[DanceswithUnix]

That's because they don't need it. Only more recent DI engines produce any noticeable particulates, hence the limit coming in, but they're easily way lower than diesel and don't need filters to meet the limits.

How many manufacturers are making DI engines? I know Alfa were making their JTS engines back in 2002, but for years they have been back to multi point as DI is hard to mix with the multi-air technology that is way more effective.

[Butcher]

How many manufacturers are making DI engines? I know Alfa were making their JTS engines back in 2002, but for years they have been back to multi point as DI is hard to mix with the multi-air technology that is way more effective.

Almost everyone makes DI engines these days. Though the latest thing is to have both DI and port injection.

[DanceswithUnix]

Almost everyone makes DI engines these days. Though the latest thing is to have both DI and port injection.

Like my old Diesel MultiJet? I can't imagine why you would do that with Petrol where you don't start off with the roughness of diesel combustion to work around, will have to try and read up on that.