Diesel Twilight?

Andrew Norton — 14 May 2020
Whatever the future holds, it's a rough voyage ahead for diesel engines in this age of emissions compliance

Diesel is experiencing a demise like that of the Holden brand. I have happy memories of simple engineering that got tangled in the unnecessarily complex world we now live in. The red and blue Holden sixes were reliable and easily fixed, just like recreational marine diesel engines when they started replacing petrol inboards in planing hulls in the sixties – diesels such as the direct injection Perkins T6-354 in Fairey Marine Huntsmans and Huntresses. 

The introduction of unleaded petrol in Australia in 1986 spelled the end of the wonderful in-line Holden sixes, just like increasing Euro and US EPA (Environmental Protection Agency) exhaust emission compliances are killing contemporary diesels.

Though we have a small population relative to land mass, diesels that are imported here have to comply with Euro and US EPA exhaust emission controls; overall, maximum torque and power are down and fuel consumption up.


The trouble with diesels is no matter how clean they appear they still produce high levels of oxides or nitrogen or Nox, leading to acid rain. Any attempt to make diesels run leaner than the usual 14.5:1 air/fuel ratio simply results in higher Nox emissions. Soot or particulate matter is another emission that causes respiratory issues. 

A step towards reducing soot emissions is opting for high pressure common rail fuel injection or unit injectors, both systems managed by electronic control units. Instead of usual injection pressures of 1500 to 5000 psi (100 to 340 bar) CR injects fuel at up to 26,000 psi or 1770 bar. The finer spray pattern encourages more thorough air/fuel burn in the combustion chambers, leading to less soot. The ECU supplies only as much fuel as is needed instead of fixed timing where the timing is too advanced below this point and too retarded above, wasting fuel. Soot emissions are dramatically reduced.

But there are other systems designed to reduce both soot and nitrogen oxides, or Nox. These systems include EGR (exhaust gas recirculation), DPF (diesel particulate filter) and Adblue with SCR. 

Even so, these 'fixes' are just that, not end solutions to make diesels as clean as petrol counterparts. Low exhaust gas temperatures produce higher soot or particulate emissions, whereas high exhaust temperatures emit more Nox. Therefore, as you’ll see, the best way to significantly reduce Nox emissions is using Adblue with SCR in conjunction with DPF.

EGR recycles between five and 35 per cent of exhaust gases through the engine. Not only does this reduce maximum torque and power outputs but also damages engines by introducing abrasive soot into the combustion chambers. This bypasses the piston rings and ends up in the sump oil where the soot can damage crankshaft, camshaft, rocker and other bearings.

DPF filters exhaust gases through a honeycomb catalyst. When this starts to clog, it’s injected with diesel, which is ignited by a small glow plug to burn off the fuel and soot before it’s passed out via the exhaust system. The burn temperature in the DPF reaches 600 degrees, but this is still low compared to catalytic converters in petrol automotive exhaust systems that run at 800 degrees.

Adblue with SCR is the latest concept, designed to replace EGR, and is most promising with regard to current emissions compliance. Adblue is a 40 per cent mix of urea (a compound found in urine) and 60 per cent deionised water which is injected into the exhaust system downstream from the DPF. The Adblue passes through a Selective Catalyst Reduction (SCR) chamber to eliminate most Nox emissions – of course, to some extent, the Nox emissions are replaced with Ammonia from the urea.

However, the Adblue can revert to its original crystalline form (before the water was added) and block injectors in freezing conditions, or when the DPF burn is taking place. If an engine's electronic management system senses that Adblue is not passing through the exhaust system the engine will go into limp mode or fail to re-start if stopped. So it's essential to keep the separate Adblue tank filled in addition to the diesel fuel tank, and check the Adblue injector at regular intervals. Peeing in the Adblue tank won't supplement the mix. And never put Adblue in the diesel tank.


Are any of these horrors fitted to recreational marine diesels? According to the major engine manufacturers I spoke with the answer is no — at this stage – although Yanmar is considering fitting Adblue with SCR on commercial diesels over 300 kW. Yanmar has managed to gain relevant Euro and US EPA emissions compliance through electronic engine management, high pressure common rail fuel injection, while opting for inter cooling in addition to turbo charging in its smaller four cylinder direct injection 'blown' engines, such as the 4JH80.

By sticking with indirect injection, which burns the air/fuel mix more thoroughly than direct injection, Nannidiesel and Westerbeke have gained Euro and US EPA compliance without reverting to electronic engine management and CR, though these engines suffer small torque and power losses. This system is only in diesels under 37 kW (49.6 brake horsepower), particularly for US EPA compliance. More powerful Nannidiesels have larger piston displacements to compensate for torque and power loss but use more fuel.

John Deere achieves US EPA emissions compliance by using either electronically controlled CR or unit injectors, without having to suffer the bolt-on nasties afflicting on-road John Deere diesels. Check out the John Deere on-road diesel website if you want to see how complex these add-ons are.

MerCruiser diesels require none of the automotive add-ons to meet US EPA Tier 3 compliance but “after treatment” devices may be needed for diesels over 600 kW to comply with EPA Tier 4 requirements.

Nannidesel uses a combination of indirect injection with mechanical control up to the N4.50 then a mix of four valve heads and CR in its more powerful Kubota-base engines. Its direct injection Toyota-based engines use CR and electronic control units. 

Westerbeke uses indirect injection with mechanical injection plus electric fuel pumps as all of the engines sold in Aus are under 37kW. 

Yanmar meets Euro and US EPA compliance through indirect injection and mechanical injection under 30kW, while direct injection engines 30kW and up use a mix of mechanical injection or CR and ECUs, depending on when the engine was first released. 


According to several diesel engine importers I spoke with, it is still possible to sell older mechanically controlled engine stock providing they met relevant emission compliance when made. Australia has no fixed emissions compliance for recreational marine diesels. But importers pointed out that stocks of older engines are rare because of the turnover of engines.

Recreationally-orientated marine diesels may be used in charter operations providing Australian Maritime Safety Authority (AMSA) is supplied with an Engine International Air Pollution Prevention (EIAPP) certificate for engines above 130 kW. So if you have a game boat with engines above 130 kW each and want to place your vessel into charter operation it must have an EIAPP certificate for each engine; the same applies for larger trawler yachts powered by a single diesel. Cats with twin sail drives don't need an ETAPP certificate but the hull must comply with relevant Australian survey standards. Such vessels are called “domestic commercial vessels”. AMSA has a full list of requirements on its website and I suggest that if you want to go down this track talk with your vessel's builder about survey compliance before taking delivery of your income earner.  

I've noticed a steady decline in the torque and power outputs of diesel engines for a given displacement over the past five-or-so years. Diesels seemed to cope with earlier emissions compliance up to Euro Stage 111, but from then on the outputs have suffered. The table in this story (two pages back) gives an indication of why this has happened. 


The future of commercial shipping is spark ignition engines running on liquid natural gas. Caterpillar showed me its early designs when I visited its Peoria Illinois manufacturing facilities in 2006 to review the then-recently released ACERT engines, while Finnish ship engine specialist Wartsila has indicated their future is the same, because soot emissions are eliminated.

Whether recreational marine diesels will eventually be phased out in favour of spark ignition LNG engines is unknown. Even if diesels are still available, I can imagine buyer resistance to having an LNG tank in the bilge compared a tank full of diesel, but Euro and US EPA compliance may dictate this.

I just hope recreational marine diesels aren't saddled with all the emissions crap that's afflicting on-road engines at the moment. If that is the case then I see a bright future for the second hand diesel engine market!  


CO is carbon monoxide, HC is hydrocarbons, Nox is oxides of nitrogen and PM is particulate matter or soot. All of these measurements are in grams per kilowatt hour.

EURO STAGE 1            

Up to 37kW19996.
37 to 75kW19995.
75 to 560kW19995.

Stage 11 was similar                    


Up to 37kW20135.04.7combined0.025
37 to 56kW20125.0
56 to 75kW20125.0
75 to 560kW20113.50.19

Note the massive reduction in PM required over Stage 1.


56 TO 130kW20145.
130 to 560kW20143.

Note the massive reduction in Nox needed to gain compliance over Stage 111.


Up to 8kW20198.07.5Combined0.4
8 to 19kW2019
19 to 37kW2019
37 to 56kW2019
56 to 130kW2019
130 to 560kW2019
560kW and above2019

Note that engines over 560 kW can emit substantially more Nox than less powerful engines.

US EPA emissions compliance is more vague but does give diesel engine manufacturers some leeway in meeting the requirements:


Up to 75kW20077.50.4


Up to 8kW2013–48.0
8 to 19kW2013–4
19 to 37kW2013–4
37kW and above2013–4

According to Yanmar Tier 4 will require a 37 per cent reduction in HC and Nox and a whopping 90 percent reduction in PM.


Feature Marine Emissions Diesel Emissions compliance Future


Andrew Norton, Supplied, Getty Images