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Pascal Hayek
TOP STORIES 
 

Marine diesel vs. gasoline

If marine technicians were to be asked about their favorite engines for marine purpose, they would definitely have different opinions as to whether they will prefer diesel or Gasoline as energy source for marine engines.

The truth is that, whether it is diesel or gasoline, each has its own advantages and disadvantages at the same time. All depends on the purpose. Is it a speed boat, a fishing boat, a pleasure yacht, a racing boat, a cruiser, a tanker or aircraft carrier.

Each type of engine is better suited for some applications than others. It all depends on your needs, the type of boat you own, and how you use it.

They are both internal combus­tion engines designed to convert the chemical energy available in fuel into mechanical energy.

In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites. It is the heat of the compressed air that lights the fuel in a diesel engine. 

Diesel's story actually begins with the invention of the gasoline engine. Nikolaus August Otto had invented and patented the gasoline engine by 1876. This invention used the four-stroke combustion principle, also known as the "Otto Cycle," and it's the basic premise for most marine engines today. 

 In its early stage, the gasoline engine wasn't very efficient, and other major methods of transportation such as the steam engine fared poorly as well. Only about 10 percent of the fuel used in these types of engines actually moved a vehicle. The rest of the fuel simply produced useless heat. 

­­I­n 1878, Rudolf Diesel was attending the Polytechnic High School of Germany when he learned about the low efficiency of gasoline and steam engines. This information inspired him to create an engine with a higher efficiency, and he devoted much of his time to developing a "Combustion Power Engine." By 1892 Diesel had obtained a patent for what we now call the diesel engine.

­When working on his calculations, Rudolf Diesel theorized that higher compression leads to higher efficiency and more power. This happens because when the piston squeezes air with the cylinder, the air becomes concentrated. Diesel fuel has high energy content, so the likelihood of diesel reacting with the concentrated air is greater. Another way to think of it is when air molecules are packed so close together, fuel has a better chance of reacting with as many oxygen molecules as possible. Rudolf turned out to be right -- a gasoline engine has a compression ratio of 8:1 to 12:1, while that of a diesel engine compresses varies between 14:1 and 25:1.

In the beginning and until recently, diesel fuel was considered dirty and sooty.

Its combustion produced lots of toxic emissions. However over the past years, vast improvements have been made on engine performance and fuel cleanliness. Direct injection devices are now controlled by advanced computers that monitor fuel combustion, increasing efficiency and reducing emissions. Better-refined diesel fuels such as ultra low sulfur diesel (ULSD) lower the amount of harmful emissions and upgrading engines to make them compatible with cleaner fuel is becoming a simpler process. Other technologies such as CRT particulate filters and catalytic converters reduce particulate matter, carbon monoxide and hydrocarbons by as much as 90 percent.  

As far as torque is concerned, diesel engines develop their maximum torque at lower rpm compared to gasoline engines. Since lower speeds mean less strain, heat and friction, these factors will translate into longer engine life, just as you will probably live longer with a regime of moderate exercise over a life of hard labor. 

However diesel engines develop a tremendous amount of heat and power at lower speeds. Failure to maintain the engine's basic systems results in engine damages far more rapidly than gas engines. In addition, since most diesel engines used nowadays are also turbocharged, this produces more heat and hence more strain on the cooling system and therefore on the engine components. 

Gasoline engines have the advantage that they are cheap to buy and cheap to repair. On the other hand diesel engines consume less fuel and live longer. 

The average marine gasoline engine runs for 1,500 hours before needing a major overhaul. The average marine diesel engine will run for more than three times that long and log an average 5,000 hours under the same conditions. The number of hours that a marine engine runs is very dependent on the amount and quality of maintenance over the years.

Although diesels can add considerable cost to a boat, they should be seriously considered because of their durability, economy of operation and safety concerns. Diesel fuel has a much higher flash point than gasoline and does not present the same threat of explosion that gasoline fumes carry.

Recently Volvo-Penta enjoyed the opportunity to conduct performance testing onboard two new, identical 10 meter Regal Express (3560) sterndrive-powered express cruisers. The gasoline-powered Regal had twin big-block gasoline engines producing 420 HP each, driving a 1.78 reduction DuoProp sterndrive with an F-5 propeller. The diesel-powered Regal was equipped with twin 285 HP Volvo-PentaKAD300 electronically controlled, 6-cylinder engines driving a 1.68:1 ratio DuoProp sterndrive and a C-6 propeller.

Test conditions were comparable and speeds were measured in two directions – into

and away from the wind with each engine operating at 3,500 RPM.

 Due to the differences in fuel, water and people on board, the diesel vessel cruise weight was 18,054# while the gasoline vessel cruise weight was only 16,476#. While the Volvo-Penta diesel-powered vessel carried 1,587# more weight and produced 32% less total horsepower,

 the results are impressive. Range figures below are calculated at 90% of total fuel tank capacity. Maximum boat speeds recorded were 53.4 MPH for the gas-powered vessel and 44.5 MPH for

 the diesel-powered vessel. However, since the majority of an operator’s time is spent at or

 below cruising RPM, the test data below is based upon approximate cruise RPM for each

propulsion package. The test report results were startling even to some very experienced boat builders and Volvo-Penta personnel.

 

 

Gasoline V-8

Volvo-Penta
KAD 300 Diesel

Result

Speed

34.1 MPH

40.9 MPH

Diesel 20% Faster

Miles / Gallon

1.2 MPG

1.7 MPG

Diesel 41.Farther

Gallons / Hour

28.1 GPH

24.3 GPH

Diesel Consumes 15% Less Fuel

Range in Miles

300 Miles

468 Miles

Diesel Travels 56% Farther

Range in Time

8.8 Hours

12.7 Hours

Diesel Travels 3.9 Hours (44%) Farther

100 Hour Consumption

2,810 gal.

2,430 gal.

Diesel Consumes 380 Gallons Less per 100 hours

 

Propulsion

MAX HP

Fuel @200 Hours

Fuel Cost / Gallon

100% Financed Payment

Fuel Costs @ 200 Hours

Monthly Savings*

Annual Savings*

Gasoline V-8

420 x 2

5,620 g

$1.99

$1,303

$11,183

 

 

VP KAD300 Sterndrive

285 x 2

4,860 g

$1.53

$1,490

$7,435

$125

$1,504

* Savings Based on fuel cost at 200 hours 


While the initial purchase price of diesel resulted in a 100% financed payment per month of $187 more, the efficiency and economy of the diesel yielded a $125-per-month savings at 200 hours per year of operation. In addition, diesel owners may anticipate lower insurance premiums, the confidence of diesel safety and durability, as well as significantly higher resale value realized by owners of well-maintained diesel boats.

Conclusion 

For high displacement marine engines which operate under heavy loads for long periods of time, the diesel engine is more reliable, while for small displacement engines which are utilized for short pleasure trips, for sports and racing purposes, definitely a gasoline engine is more appropriate.

Pascal Johnny Hayek B. Eng. AUB, 1983 Service Manager Porsche Service Centre, Kuwait

 

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