Do diesel engines actually run lean?

Many modern diesel engines don't have throttles while some do. This absence leaves many blue-singlet wearers confused. You know who you are…

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So, diesel engines actually run lean, right?

The absence of a throttle leads many people to wrongly conclude - because the engine runs wide-open on air - that these throttle-less diesels always run lean. But are they right?

I may be a bit behind the times diesel technology-wise, but I think diesels pretty much always ran lean, as there's no obstruction or butterfly valve in the intake side of things. 

This would mean diesel engines always run lean - unless you're working the engine hard, in which case you're close to max fuel delivery and thus close to stoichiometry - so there should always be extra oxygen for the DPF to do its regen voodoo when the engine's under low load.

Please correct me if I'm wrong (or just out of date.)

- Aussiebloke609

Yeah, that’s definitely wrong, twice. So, points for consistency. 

Image: Diesel Care Australia/Twitter

A lot of people suggested this. So let’s talk about that. 

In theory, diesels don’t need throttles because performance is regulated by fuel delivery, and they run wide-open (as it were) on air. More fuel equals more performance; less fuel means less performance. Air intake: unrestricted. That’s diesel, in theory. And, in practice for some engines.

But quite a few modern diesels actually have throttles that are ECU-controlled. And what that does is make it easier to manage the exhaust gas recirculation system. If you introduce a vacuum into the inlet air plumbing by closing the throttle that makes it easier to increase the volume of EGR. Less competition for flow, right?

And I know there’s going to be comments saying what a bad thing EGR is. ‘Just delete it mate - it’s shit.’ If you want everyone to know what a Muppet you are, just knock yourself out. It’s a free country.

EGR makes engines more efficient and it reduces emissions. So, it’s a good thing, on balance, provided the engineers in R&D did their jobs right. And removing it is illegal. So there’s that.

If you’re in the market for a new ute in 2020, here’s my Ultimate Ute Market Buyer’s Guide to point you in the right direction >>

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Lean a lesson

This notion that ‘no throttle’ equals ‘always running lean’ is just insane.

If you run a diesel engine too lean, temperatures spike and NOx emissions go through the roof. Just ask Volkswagen. They did that experiment, globally. It was in all the newspapers. Fuel economy increases (good), performance increases (good), the air gets more toxic (bad).

Just to dive a bit deeper into this ‘lean/rich’ thing. Diesels can run rich or lean despite having no throttle. The mixture is completely unrelated to the presence or absence of a throttle.

In a modern direct injection diesel, the thing that enters the cylinder on the inlet stroke is air (sometimes mixed with EGR). Fuel does not come in on the inlet stroke. The inlet valve eventually closes. Then there’s a compression stroke. Up near the top of the compression stroke, you’ve got a charge of hot, compressed air (and maybe some EGR).

At about that time, the injector fires off a series of precisely metered fuel delivery pulses, which spontaneously ignite. That’s why they call it a ‘compression ignition’ engine. The amount of fuel is precise.

The thing that determines the richness or lean-ness of the mixture is how much fuel the injectors deliver - and that’s entirely up to the dudes who write the code for the engine control ECU. They can write it lean, for particular operating conditions, and rich for others and stoichiometric for others. It depends on the conditions and the objective.

The absence of a throttle has absolutely no bearing on the amount of fuel in the mixture. If you got lost after the second paragraph there, here’s my dummy’s guide to combustion >>

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Intake a breath

Four-wheel driver Barry (not Crocker, as far as I’m aware) sent me this recently, as he argues the toss, while sinking a few, about which way to point his equipment in public.

Definitely not at the kids, and especially not at the local feminist at the bar - you never know if they’re carrying anything sharp.

I've recently fitted a snorkel to my 80 series.

While dismantling the Toyota intake plumbing it seemed at least to me that they go to a lot of trouble to avoid water droplets getting into the air filter.

So when looking at the design of my Safari snorkel it seems to be severely lacking in similar design.

I am now chastised for having my snorkel intake facing to the rear! 

I am told I will lose performance by not getting the "ram air" effect. This has not swayed me in any way as I believe ram air does not have any effect until speeds exceed 160km/hr but water getting into the engine may have a detrimental effect on my diesel.

Good pub argument material I frequently lose, am I right or wrong?

- Barry

Barry, Barry, Barry - it’s fair to say you’re both wrong,mate.

The ram air effect is bullshit.

Even in racing the purpose of a forward-facing scoop is overwhelmingly to swallow cold air - not as a form of de facto forced induction. 

Your 80 Series is turbocharged. So, unless you introduce a restriction into the inlet air plumbing, what you do on the upstream end of the inlet plumbing (upstream of the turbo) will have no tangible effect on performance. 

A little bit of rain getting in could even be beneficial mate. Because turbos are so hot, the latent heat of vapourisation of the water would increase the density of the air as it gets pumped up in the turbo. And that’s not going to be a bad thing.

It’s hardly as if you’re going to hydraulic the engine with rain.

Face the air intake any way you want - it’s not going to make any difference and being chastised by mates in the pub - you love it. We all do. Aside from boobies, what else is there?

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In dislike Flint

Then, typical of this day and age - where the prudes of the world think it’s in the best interests of everybody else they project their own social insecurities onto an unknown populous and use it to construct a flaky argument against this fine channel - I got this.

Your research of the facts and your analysis are excellent.

But IMHO your presentations are diminished by the crude, constant references to dicks, dildos and your references to and demonstrations of sexual acts as metaphors for business bastardry. 

It's so over the top it's not funny and the crudity overwhelms your otherwise incisive commentary and becomes uncomfortable to watch. And at times it sounds sexist and demeaning to women.

Maybe you think you're catering to your target audience? It's not only blokes that buy cars, Jack.

- Alan Flint

Really? My cock and I repudiate your allegations without reservation. Especially my cock. He takes that kind of thing very seriously. Me? I’m more a ‘live and let live’ kinda dude, even when it comes to arseholes. 

Now, look, I know I’m getting older but I cannot remember the last time I actually demonstrated a sexual act on this fine channel, and I really do think I’d remember a lapse of editorial judgement as monumental as that.

I’m surprised you left out boobies, Alan. Because I mention and depict them rather a lot.

As to the phrase: “sexist and demeaning to women” - that’s almost tautological and certainly redundant because in our fucked-up and increasingly woke society I’d suggest it’s impossible to say anything that could be criticised as being sexist and demeaning towards men, even though this would clearly be an example of true gender equality. Which does not yet exist, mainly as a consequence of latent, unacknowledged discrimination against men. So there’s that.

I actually get quite a few e-mails from chicks who tell me they enjoy the reports - because at least I’m authentic and not one of these dickless metrosexual Muppets who might as well be about as androgynous as a shop-front mannequin. 

These female correspondents on the channel are obviously women who realise that a big chunk of these reports is satirical in nature. It pains me to point this out to dipshits, however necessarily. You might want to look that up mate. Satire.

But just for disambiguation - your overall argument is bullshit. It’s a classic straw man bullshit argument. Because if you take satire literally, it becomes an easy proposition to defeat. Much more easy to defeat than the points I am really making in my reports, arsehole, so, with all due respect, which would be none, piss off and watch something else. In time, I assure you, I will get over the trauma stemming from the void left in my life, caused by your absence from the audience.

Finally, Alan, I’d suggest: try getting laid, mate. Potentially even with a woman. (But not the inflatable kind. Pneumatic Nancy.) That might ratchet the up-tightedness down a few notches. To the benefit of us all. It usually does with me, however temporarily.

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