My petrol car seems to behave like a diesel. I have just gone up a
hill in third gear that is too steep to cycle up. Does this suggest
unusual gearing ratios? (Or bad driving on my part!)
The car is a Nissan MIcra 1.2 - K12 series.
Petrol engines are generally *more* flexible than diesels. A diesel
that produces a torque of x at 2000rpm is not in the slightest any
better than a petrol engine that will produce a torque of x/2 at
The petrol engine will likely produce some useful torque at 2000rpm
and meybe even less than that, the diesel will likely not produce much
at 1000rpm. Thus the petrol engine will require less gear changing
than the diesel.
Modern diesels are improving but it still easier to get a petrol
engine with a nice flat torque characteristic than a diesel.
On 05/08/2018 11:42, firstname.lastname@example.org wrote:
I've driven various modern(ish) turbo diesels and find they have a
narrow power band to add to their turbo lag.
My wife's car and mine are both petrol, and will give usable torque at
1500RPM, and rev to 7500.
On Sunday, 5 August 2018 22:38:34 UTC+1, Vir Campestris wrote:
The current diesel can be driven below idle speed even in 4th, picking up speed slowly. I've not had a petrol that does that. If you have to go up to 1500rpm to get anything usable that's way less low end torque.
On 05/08/2018 22:55, email@example.com wrote:
What's "below idle" and what's the red line?
Our cars have a good usable range 7500/1500 = 5 fold. And, yes, there is
torque below that. But it's not good for the engine, nor is it
efficient, so I don't do it.
On Tuesday, 7 August 2018 21:57:20 UTC+1, Vir Campestris wrote:
idle speed is 800rpm. Red line I forget, anything from 5k to nearly 6k. Will check if I remember.
On the contrary it's more efficient & better for the engine.
Efficiency: you lose out on valve timing, which is way off at very low revs. But you gain on cylinder fill, which has an even larger gain than valve timing's loss. Driving it like that gets me rather better than mfr mpg figures.
Life expectancy: If you look at engine failure modes, failures are usually much more to do with total number of revs than load. Increasing load & reducing revs makes engines last longer.
The ford engine does, I've driven the 2.7V6 fitted to jaguars, and that
has been tamed a LOT to give decent torque band but the TD4 firred to
MkII freelanders is vile - a ltttle netter than the mindeo engine but
still vbery peaky.
It all depends on the turbo. Vraiable geonytery turbos or twin tiurbose
can do w huge amount,.
The earlier BMW turbodiesel fitted to MkI Freelanders is
excerptionally flat torqiue wise but delivers less power.
That is very true if the engine has variable valve timimg and decent
ports and so on
Back in the day it wasn't a great idea to produce an engine that was
still producing torque at 7500 because some klutz would then rev it to 8000.
'sports' engines would do that and camne with revcounters to show you
when to change up.
fast road cams would imnprove the top end at the expense of low down
torque and a lumpy idle
With VVT and rev limiters on many engines now there is no need to
engineer in 'rev limiting by loss of torque die to bad breathing'
Nissan and Toyota are pioneers of this sirt of technology.
Turbodiesel performmance is really mostly about turbo design with
variable geometry turbos making for a much broader response curve. Some
manufacturers dont use it, and it shows with nasty narrow peaky
performance. But a well implemented design - the Jaguar Ford V6 will
pull from less than 1000 to a red line at 6250.
Petrol performance on normally aspirated enginines is all aboyt
breathing and recent advances in variable valve timing as well as of
course electronic ignition/fuel injection enable the engine to be
optimised over a far wider rev band than used to be the case.
I've driven an old Almera that pulled from around 1200 to 8000rpm.
Astonishing engine for a 'shopping trolley'.
Engines get better.
Cars? Not so much
The New Left are the people they warned you about.
Are there two types of diesel engine? The old buses had engines that
ran very slowly with a lot of torque and a deep sound. New diesels
seem to run much faster sometimes with an almost screeching sound.
Well in the bad old days you got power out of a diesel by making it BIG
and hence it wasn't great at revving. Mechanical fuel injection led to
pretty poor and smoky exhausts
The along came the turbo and electronic fuel injection, and various
advances in fuel injection like common rail etc etc.
Net result, smaller engine for same power and more revs available and
better fuel consumption - and less 'clatter'
“Progress is precisely that which rules and regulations did not foresee,”
You are of course WRONG!
Diesel driving involves much less gear changing than the narrow torque
curve of a petrol engine. My modern BMW diesel has more than adequate
torque even below 1000rpm. cc for cc, a diesel produces more power than
petrol. It is usually considered that a 2L modern diesel engine equates
to a 3L petrol.
Absolutely not so.
Like for like, no diesel produces as much peak power (BHP) as petrol.
The torque output may be very different, with diesels usually producing
more at low revs.
The big difference is most diesels are turbos. Which gives the effect of a
larger engine. But it does exactly the same to a petrol engine too.
A state of the art petrol engine with turbo will out perform a state of
the art diesel with turbo if both are the same capacity.
*I'd kill for a Nobel Peace Prize *
Dave Plowman firstname.lastname@example.org London SW
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