Is it still a
good idea – from an economic point-of-view – to buy a diesel-powered car?
The answer’s no
longer as clearcut as it used to be, when the answer was an unqualified yes.
When diesel engines were simpler, cheaper to operate and more durable than gas
engines.
When their
mileage was much better – and diesel fuel much cheaper.
Diesel engines
had fewer parts; they were (and still are) compression-ignition engines, so no
spark plugs to wear out and and tune-ups were a non-issue because there
wasn’t really anything to tune. As long as the injector pump worked, the engine
usually ran.
And it ran for
a long time.
Hundreds of
thousands of miles, usually without needing major work during that time. This
longevity was a function of both simplicity (fewer parts, less to go wrong over
time) and the necessarily stouter design of the diesel engine itself, which had
to have a beefy (usually heavy cast iron) block and so on to handle the
stresses of very high compression.
Diesel fuel
cost less, too. Chiefly because it was a less-refined fuel. It cost less to
make than gasoline. So it sold for less than gasoline.
And a
diesel-powered vehicle used to use a lot less fuel vs. a gas-engined
equivalent. Plus (and it was a big plus) you could use almost any type of
diesel, including “homebrew” bio-diesel and even waste vegetable oil).
These factors
quickly made up for the somewhat (but not hugely) higher up-front costs you
paid to buy the diesel engine.
Today, the
economics are much iffier.
First, the
up-front cost are much higher. This is a function of the additional hardware –
mostly emissions-related – that modern diesels have to have in order to
be legal to sell. Diesel-powered cars generally must have urea injection and
particulate traps and these add a layer of maintenance and repair costs to
diesel ownership, in addition to higher costs up front. If the car has urea
injection, one must regularly refill the urea tank – a small but not irrelevant
cost and additional hassle.
And modern, EPA
fatwa-friendly diesel engines are more rather than less complicated. They are
also more finicky about the fuel they’re fed, which (generally) must be low or
“ultra low” sulfur diesel, which is more expensive because it now costs more to
make it than gasoline. You generally can’t use inexpensive, home-brewed fuel,
either. At least, not legally. And probably not without violating the terms and
conditions of your warranty coverage.
The mileage
isn’t fantastic, either.
Many new
gas-powered small cars (and some mid-sized cars) are capable of almost 40 MPG
on the highway; the handful of diesel-powered cars you can currently buy are
capable of doing better, but not spectacularly.
Example: one of
the few diesel-powered cars not fatwa’d off the market is the Chevy Cruz
diesel, which could go 27 MPG in city driving and 46 MPG on the highway. This
is very good. But a gas-engined Mazda6 sedan can go 25 MPG in city driving and
37 MPG on the highway – which is nearly as good. And the Mazda’s price –
$21,495 – is much better than the price of the Chevy – $25,660 for the 2015
model (which is the last year this model was available; Chevy also had to pull
it off the market because the ’15, while “clean” by any sane standard, isn’t
“clean” enough for Uncle’s current not-sane standards).
If you bought
the Chevy over the Mazda, you’d have spent $4,020 to save 2 MPG on fuel in city
driving and 9 MPG on the highway. Average it out, it’s about 7 MPG.
Call it ten.
It still
doesn’t make much, if any, economic sense to buy the diesel.
It might, if
the gas-engined Mazda’s lifespan was much lower – as used to be the case, gas
engine vs. diesel engine. But it’s not the case today (and hasn’t been, for
many years). It’s routine for gas-engined cars to run reliably for several
hundred thousand miles – as opposed to maybe one hundred thousand miles, as was
the case once upon a time.
A diesel engine
may still out-last a gas engine, but after 20 years and 200,000 or so miles,
who’s counting?
And modern
diesel engines – more complex, not as maintenance-free – will probably not last
as long as their forbears and will almost certainly cost more to keep going
over time (see the part above about the urea injection systems and particulate
traps).
Meanwhile,
gasoline costs less than diesel – which further undermines the economic case
for the diesel.
Even the
performance case is not what it once was.
Diesels used to
have a huge advantage, torque-wise. They made a lot of it – and made it early,
low down in the powerband. This gave the vehicle strong performance
accelerating from a standstill and also in the mid-ranges; it wasn’t necessary
to “floor it” to get a diesel-powered vehicle to move with authority.
But gas engines
do that now, too.
In part,
because – like diesels – they tend to be turbocharged. And these turbos are not
the peaky (and laggy) turbos of the past. They are twin-scroll and
multiple/staged turbos that build boost almost immediately, without lag – and
very much like a turbo-diesel. Take a look at the specs of any of several
currently available turbo-gas powered cars. They offer big torque (and peak
torque) at barely-faster-than-idle RPM speed (in some cases, as low as 1,200
RPM but generally, most of them make their peak torque before 2,000 RPM… just
like a turbo-diesel).
And they make
horsepower, too.
Many diesels
are deficient on that score; they pull hard initially – but generally haven’t
got much on tap higher in the power band. They are down-low engines. But modern
turbo-gas engines are good down-low and up high.
So, is there
any good reason to consider buying a diesel engine?
If you need to
pull/tow a heavy load then a diesel is still a good choice. This is why diesel
engines are still readily available in pick-ups trucks, especially big ones.
Which also have big diesels (such as the Cummins turbo-diesel V8 used in the
new Nissan Titan XD) that make superhero torque.
If you like
long legs. A diesel-powered Mercedes E-Class ($52,650) isn’t exactly
economical, but it can take you almost 900 miles (886.2 to be precise) on a
full tank, highway miles. This is a function of the diesel’s being in its
element on the highway and also of the large (21.2 gallon) fuel tank the car
has.
It’s worth
noting that while diesels are almost unavailable in modestly priced cars (VW
had to stop selling them, per Uncle’s fatwa – and Chevy had to temporarily/”voluntarily”
pull the Cruz diesel until it can be made Uncle-compliant) you can still find
diesels under the hoods of high-end (and high-priced) stuff.
Precisely
because economic considerations are no longer the main consideration for
considering a diesel engine.
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